
Gass 



Book. 



COPYRIGHT DEPOSIT 




:4 




Pr. I. OXE OF THE LiXES OF ToWERS AT RADIO .CeXTRAL 

( Courtesy of Radio Corporatiox of America ) . 



LETTERS OF 

A RADIO-ENGINEER 

TO HIS SON 



BY 

JOHN MILLS 

Engineering Department, Western Electric Company, Inc., 

Author of "Radio-Communication," "The Realities of 

Modern Science," and "Within the Atom" 



m 



NEW YORK 
HARCOURT, BRACE AND COMPANY 



^^'% 



COPYRIGHT, 1922, BY 
HABCOURT, BRACE AND COMPANY, INC. 



ED IN THE U S A. BY * 



THE QUINN a BODEN COMPANY 
RAHWAY N J 

Sl:P12'22 \^ 

©C1.A683176 



vc 



TO 
J. M., Jr. 



CONTENTS 

LETTER PAGE 

1 Electricity and Matter 3 

2 Why a Copper Wire Will Conduct Elec- 

tricity 9 

3 How A Battery Works 16 

4 The Batteries in Your Radio Set ... 27 

5 Getting Electrons from a Heated Wire . . 34 

6 The Audion 40 

7 How TO Measure an Electron Stream . . 48 

8 Electron-Moving-Forces 57 

9 The Audion-Characteristic . . . . . 66 

10 Condensers and Coils 77 

11 A ^^C-W" Transmitter 86 

12 Inductance and Capacity 96 

13 Tuning 112 

14 Why and How to Use a Detector . . . 124 

15 Radio-Telephony 140 

16 The Human Voice 152 

17 Grid Batteries and Grid Condensers for De- 

tectors 165 

18 Amplifiers and the Regenerative Circuit . 176 

19 The Audion Amplifier and Its Connections . 187 

20 Telephone Receivers and Other Electro- 

magnetic Devices 199 

21 Your Receiving Set and How to Experiment 211 

22 HiGH-PowERED RxVdio-Telephone Transmitters 230 

23 Amplification at Intermediate Frequencies . 242 

24 By Wire and by Radio 251 

Index • 263 



LIST OF PLATES 

I One of the Lines of Towers at 

Eadio Central .... Frontispiece 

PAGE 

II Bird's-Eye View of Eadio Central . . 10 

III Dry Battery for Use in Audion Circlets, 

AND ALSO Storage Battery .... 27 

IV Radiotron 42 

V Variometer and Variable Condenser op the 

General Radio Company. Voltmeter 
and Ammeter of the Weston Instru- 
ment Company 91 

VI Low-Power Transmitting Tube, U V 202 . 106 
VII Photographs of Vibrating Strings . . 155 
VIII To Illustrate the Mechanism for the Pro- 
duction OF the Human Voice . . . 170 
IX Western Electric Loud Speaking Receiver. 
Crystal Detector Set of the General 
Electric Co. Audibility Meter of Gen- 
eral Radio Co 203 

X Audio-Frequency Transformer and Banked- 

WouND Coil 218 

XI Broadcasting Equipment, Developed by the 
American Telephone and Telegraph 
Company and the Western Electric 

Company 235 

XII Broadcasting Station of the American 
Telephone and Telegraph Company on 
THE Roof of the Walker-Lispenard Bldg. 
IN New York City where the Long-dis- 
tance Telephone Lines Terminate . . 250 



LETTERS OF A RADIO- 
ENGINEER TO HIS SON 



I 



LETTER 1 
ELECTRICITY AND MATTER 

My Deae Son : 

You are interested in radio-telephony and want 
me to explain it to you. I'll do so in the shortest 
and easiest way which I can devise. The explana- 
tion will be the simplest which I can give and still 
make it possible for you to build and operate your 
own set and to understand the operation of the 
large commercial sets to which you will listen. 

Ill write you a series of letters which will con- 
tain only what is important in the radio of to-day 
and those ideas which seem necessary if you are to 
follow the rapid advances which radio is making. 
Some of the letters you will find to require a second 
reading and study. In the case of a few you might 
postpone a second reading until you have finished 
those which interest you most. I'll mark the letters 
to omit in this way. 

All the letters will be written just as I would talk 
to you, for I shall draw little sketches as I go along. 
One of them will tell you how to experiment for 
yourself. This will be the most interesting of all. 
You can find plenty of books to tell you how radio 
sets operate and what to do, but very few except 
some for advanced students tell you how to experi- 
ment for yourself. Not to waste time in your o^vn 

3 



4 LETTERS OF A RADIO-ENGINEER 

experiments, however, you will need to be quite 
familiar with the ideas of the other letters. 

What is a radio set? Copper wires, tinfoil, 
glass plates, sheets of mica, metal, and wood. Where 
does it get its ability to work — that is, where does 
the ''energy'' come from which runs the set? From 
batteries or from dynamos. That much you know al- 
ready, but what is the real reason that we can use 
copper wires, metal plates, audions, crystals, and bat- 
teries to send messages and to receive them! 

The reason is that all these things are made of 
little specks, too tiny ever to see, which we might 
call specks of electricity. There are only two kinds 
of specks and we had better give them their right 
names at once to save time. One kind of speck is 
called ''electron'' and the other kind "proton." 
How do they differ? They probably differ in size 
but we don't yet know so very much about their 
sizes. They differ in laziness a great deal. One is 
about 1845 times as lazy as the other. That is, it 
has eighteen hundred and forty-five times as much 
inertia as the other. It is harder to get it started 
but it is just as much harder to get it to stop after 
it is once started or to change its direction and go 
a different direction. The proton has the larger 
inertia. It is the electron which is the easier to 
start or stop. 

How else do they differ? They differ in their 
actions. Protons don't like to associate with other 
protons but take quite keenly to electrons. And 
electrons — they go with protons but they won't as- 



ELECTRICITY AND MATTER 5 

sociate with each other. An electron always likes 
to be close to a proton. Two is company when one 
is an electron and the other a proton but three is a 
crowd always. 

It doesn't make any difference to a proton what 
electron it is keeping company with provided only 
it is an electron and not another proton. All elec- 
trons are alike as far as we can tell and so are all 
protons. That means that all the stuff, or matter, 
of our world is made up of two kinds of building 
blocks, and all the blocks of each kind are just 
alike. Of course you mustn't think of these blocks 
as like bricks, for we don't know their shapes. 

Then there is another reason why you must not 
think of them as bricks and that is because when 
you build a house out of bricks each brick must rest 
on another. Between an electron and any other 
electron or between two protons or between an 
electron and a proton there is usually a relatively 
enormous distance. There is enough space so that 
lots of other electrons or protons could be fitted in 
between if only they were willing to get that close 
together. 

Sometimes they do get very close together. I 
can tell you how if you will imagine four small boys 
playing tag. Suppose Tom and Dick don't like to 
play with each other and run away from each other 
if they can. Now suppose that Bill and Sam won't 
play with each other if they can help it but that 
either of them will play with Tom or Dick whenever 
there is a chance. Now suppose Tom and Bill see 



6 LETTERS OF A RADIO-ENGINEER 

each other; they start running toward each other 
to get up some sort of a game. But Sam sees Tom 
at the same time, so he starts running to join him 
even though Bill is going to be there too. Mean- 
while Dick sees Bill and Sam running along and 
since they are his natural playmates he follows 
them. In a minute they are all together, and play- 
ing a great game, although some of the boys don't 
like to play together. 

Whenever there is a group of protons and elec- 
trons playing together we have what we call an 
**atom.'' There are about ninety different games 
which electrons and protons can play, that is ninety 
different kinds of atoms. These games differ in the 
number of electrons and protons who play and in 
the way they arrange themselves. Larger games 
can be formed if a number of atoms join together 
Then there is a ^'molecule.'' Of molecules there 
are as many kinds as there are different substances 
in the world. It takes a lot of molecules together 
to form something big enough to see, for even the 
largest molecule, that of starch, is much too small 
to be seen by itself with the best possible microscope. 

What sort of a molecule is formed will depend 
upon how many and what kinds of atoms group to- 
gether to play the larger game. Whenever there is 
a big game it doesn't mean that the little atomic 
groups which enter into it are all changed around. 
They keep together like a troop of boy scouts in a 
grand picnic in which lots of troops are present. 
At any rate they keep together enough so that we 



ELECTRICITY AND MATTER 7 

can still call them a group, that is an atom, even 
though they do adapt their game somewhat so as to 
fit in with other groups — that is with other atoms. 

What will the kind of atom depend upon ! It will 
depend upon how many electrons and protons are 
grouped together in it to play their little game. 
How any atom behaves so far as associating with 
other groups or atoms will depend upon what sort of 
a game its own electrons and protons are playing. 

Now the simplest kind of a game that can be 
played, and the one with the smallest number of elec- 
trons and protons, is that played by a single proton 
and a single electron. I don't know just how it is 
played but I should guess that they sort of chase 
each other around in circles. At any rate I do know 
that the atom called ^' hydrogen *' is formed by just 
one proton and one electron. Suppose they were 
magnified until they were as large as the moon and 
the earth. Then they would be just about as far 
apart but the smaller one would be the proton. 

That hydrogen atom is responsible for lots of in- 
teresting things for it is a great one to join with 
other atoms. We don't often find it by itself al- 
though we can make it change its partners and go 
from one molecule to another very easily. That 
is what happens every time you stain anything with 
acid. A hydrogen atom leaves a molecule of the acid 
and then it isn't acid any more. What remains isn't 
a happy group either for it has lost some of its play- 
fellows. The hydrogen goes and joins with the stuft 
which gets stained. But it doesn't join with the 



8 LETTERS OF A RADIO-ENGINEER 

whole molecule; it picks out part of it to associate 
with and that leaves the other part to take the place 
of the hydrogen in the original molecule of acid 
from which it came. Many of the actions which 
we call chemistry are merely the result of such 
changes of atoms from one molecule to another. 

Not only does the hydrogen atom like to associate 
in a larger game with other kinds of atoms but it 
likes to do so with one of its own kind. "When it 
does we have a molecule of hydrogen gas, the same 
gas as is used in balloons. 

We haven't seemed to get very far yet toward 
radio but you can see how we shall when I tell you 
that next time I shall write of more complicated 
games such as are played in the atoms of copper 
which form the wires of radio sets and of how these 
wires can do what we call ' ' carrying an electric cur- 
rent. '^ 



LETTER 2 

WHY A COPPER WIRE WILL CONDUCT 
ELECTRICITY 

My Deae Young Atomist: 

You have learned that the simplest group which 
can be formed by protons and electrons is one pro- 
ton and one electron chasing each other around in a 
fast game. This group is called an atom of hydro- 
gen. A molecule of hydrogen is two of these groups 
together. 

All the other possible kinds of groups are more 
complicated. The next simplest is that of the atom 
of helium. Helium is a gas of which small quantities 
are obtained from certain oil wells and there isn^t 
very much of it to be obtained. It is an inert gas, as 
we call it, because it wonH burn or combine with any- 
thing else. It doesn't care to enter into the larger 
games of molecular groups. It is satisfied to be 
as it is, so that it isn't much use in chemistry be- 
cause you can't make anything else out of it. That's 
the reason why it is so highly recommended for 
filling balloons or airships, because it cannot burn 
or explode. It is not as light as hydrogen but it 
serves quite well for making balloons buoyant in air. 

This helium atom is made up of four electrons and 
four protons. Right at the center there is a small 
closely crowded group which contains all the pro- 



10 LETTERS OF A RADIO-ENGINEER 

tons and two of the electrons. The other two elec- 
trons play around quite a little way from this inner 
group. It will make our explanations easier if we 
learn to call this inner group "the nucleus" of the 
atom. It is the center of the atom and the other two 
electrons play around about it just as the earth and 
Mars and the other planets play or revolve about the 
sun as a center. That is why we shall call these 
two electrons ''planetary electrons.'' 

There are about ninety different kinds of atoms 
and they all have names. Some of them are more 
familiar than hydrogen and helium. For example, 
there is the iron atom, the copper atom, the sulphur 
atom and so on. Some of these atoms you ought 
to know and so, before telling you more of how 
atoms are formed by protons and electrons, I am go- 
ing to write down the names of some of the atoms 
which we have in the earth and rocks of our world, 
in the water of the oceans, and in the air above. 

Start first with air. It is a mixture of several 
kinds of gases. Each gas is a different kind of atom. 
There is just a slight trace of hydrogen and a very 
small amount of helium and of some other gases 
which I won't bother you with learning. Most of 
the air, however, is nitrogen, about 78 percent in 
fact and almost all the rest is oxygen. About 20.8 
percent is oxygen so that all the gases other than 
these two make up only about 1.2 percent of the 
n/mosphere in which we live. 

The earth and rocks also contain a gTeat deal of 
oxygen ; about 47.3 percent of the atoms which form 



WHY A WIRE CONDUCTS 11 

earth and rocks are oxygen atoms. About half of 
the rest of the atoms are of a kind called silicon. 
Sand is made up of atoms of silicon and oxygen 
and you know how much sand there is. About 27.7 
percent of the earth and its rocks is silicon. The 
next most important kind of atom in the earth is 
aluminum and after that iron and then calcium. 
Here is the way they run in percentages : Aluminum 
7.8 percent; iron 4.5 percent; calcium 3.5 percent; 
sodium 2.4 percent ; potassium 2.4 percent ; magnes- 
ium 2.2 percent. Besides these which are most im- 
portant there is about 0.2 percent of hydrogen and 
the same amount of carbon. Then there is a little 
phosphorus, a little sulphur, a little fluorine, and 
small amounts of all of the rest of the different kinds 
of atoms. 

Sea water is mostly oxygen and hydrogen, about 
85.8 percent of oxygen and 10.7 percent of hydrogen. 
That is what you would expect for water is made up 
of molecules which in turn are formed by two atoms 
of hydrogen and one atom of oxygen. The oxygen 
atom is about sixteen times as heavy as the hydrogen 
atom. However, for every oxygen atom there are 
two hydrogen atoms so that for every pound of 
hydrogen in water there are about eight pounds of 
oxygen. That is why there is about eight times as 
high a percentage of oxygen in sea water as there is 
of hydrogen. 

Most of sea water, therefore, is just water, that is, 
pure water. But it contains some other substances 
as well and the best known of these is salt. Salt is a 



12 LETTERS OF A RADIO-ENGINEER 

substance the molecules of which contain atoms of 
sodium and of chlorine. That is why sea water is 
about 1.1 percent sodium and about 2.1 percent 
chlorine. There are some other kinds of atoms in 
sea water, as you would expect, for it gets all the 
substances which the waters of the earth dissolve 
and carry down to it but they are unimportant in 
amounts. 

Now we know something about the names of the 
important kinds of atoms and can take up again the 
question of how they are formed by protons and 
electrons. No matter what kind of atom we are deal- 
ing with we always have a nucleus or center and 
some electrons playing around that nucleus like tiny 
planets. The only differences between one kind of 
atom and any other kind are diiferences in the nu- 
cleus and differences in the number and arrange- 
ment of the planetary electrons which are playing 
about the nucleus. 

No matter what kind of atom we are considering 
there is always in it just as many electrons as pro- 
tons. For example, the iron atom is formed by a 
nucleus and twenty-six electrons playing around it. 
The copper atom has twenty-nine electrons as tiny 
planets to its nucleus. What does that mean about 
its nucleus! That there are twenty-nine more pro- 
tons in the nucleus than there are electrons. Silver 
has even more planetary electrons, for it has 47. 
Radium has 88 and the heaviest atom of all, that of 
aranium, has 92. 

We might use numbers for the different kinds of 



WHY A WIRE CONDUCTS 13 

atoms instead of names if we wanted to do so. We 
could describe any kind of atom by telling how many 
planetary electrons there were in it. For example, 
hydrogen would be number 1, helium number 2, lith- 
ium of which you perhaps never heard, would be 
number 3, and so on. Oxygen is 8, sodium is 11, 
chlorine is 17, iron 26, and copper 29. For each kind 
of atom there is a number. Let's call that number 
its atomic number. 

Now let's see what the atomic number tells us. 
Take copper, for example, which is number 29. In 
each atom of copper there are 29 electrons playing 
around the nucleus. The nucleus itself is a little 
inner group of electrons and protons, but there are 
more protons than electrons in it ; twenty-nine more 
in fact. In an atom there is always an extra proton 
in the nucleus for each planetary electron. That 
makes the total number of protons and electrons the 
same. 

About the nucleus of a copper atom there are 
playing 29 electrons just as if the nucleus was a 
teacher responsible for 29 children who were out 
in the play yard. There is one very funny thing 
about it all, however, and that is that we must think 
of the scholars as if they were all just alike so that 
the teacher couldn't tell one from the other. Elec- 
trons are all alike, you remember. All the teacher 
or nucleus cares for is that there shall be just the 
right number playing around her. You could bring 
a boy in from some other play ground and the teacher 
couldn't tell that he was a stranger but she would 



14 LETTERS OF A RADIO-ENGINEER 

know that something was the matter for there would 
be one too many in her group. She is responsible for 
just 29 scholars, and the nucleus of the copper atom 
is responsible for just 29 electrons. It doesn't make 
any difference where these electrons come from pro- 
vided there are always just 29 playing around the 
nucleus. If there are more or less than 29 some- 
thing peculiar will happen. 

We shall see later what might happen, but first 
let's think of an enormous lot of atoms such as there 
would be in a copper wire. A small copper wire 
will have in it billions of copper atoms, each with its 
planetary electrons playing their invisible game 
about their own nucleus. There is quite a little dis- 
tance in any atom between the nucleus and any of the 
electrons for which it is responsible. There is usu- 
ally a greater distance still between one atomic group 
and any other. 

On the whole the electrons hold pretty close to 
their own circles about their own nuclei. There is 
always some tendency to run away and play in some 
other group. With 29 electrons it's no wonder if 
sometimes one goes wandering off and finally gets 
into the game about some other nucleus. Of course, 
an electron from some other atom may come wan- 
dering along and take the place just left vacant, so 
that nucleus is satisfied. 

We don't know all we might about how the elec- 
trons wander around from atom to atom inside a 
copper wire but we do know that there are always 
a lot of them moving about in the spaces between 



WHY A WIRE CONDUCTS 15 

the atoms. Some of them are going one way and 
some another. 

It's these wandering electrons which are affected 
when a battery is connected to a copper wire. Every 
single electron which is away from its home group, 
and wandering around, is sent scampering along 
toward the end of the wire which is connected to the 
positive plate or terminal of the battery and away 
from the negative plate. That's what the battery 
does to them for being away from home; it drives 
them along the wire. There's a regular stream or 
procession of them from the negative end of the 
wire toward the positive. When we have a stream 
of electrons like this we say we have a current of 
electricity. 

We'll need to learn more later about a current of 
electricity but one of the first things we ought to 
know is how a battery is made and why it affects 
these wandering electrons in the copper wire. 
That's what 1 shall tell you in my next letter.^ 

1 The reader who wishes the shortest path to the construction and 
operation of a radio set should omit the next two letters. 



LETTER 3 
HOW A BATTERY WORKS 

(This letter may be omitted on the first reading.) 

My Deae Boy: 

When I was a boy we used to make our own bat- 
teries for our experiments. That was before stor- 
age batteries became as widely used as they are to- 
day when everybody has one in the starting system 
of his automobile. That was also before the day of 
the small dry battery such as we use in pocket flash 
lights. The batteries which we made were like those 
which they used on telegraph systems, and were 
sometimes called '^gravity" batteries. Of course, 
we tried several kinds and I believe I got quite a 
little acid around the house at one time or another. 
I'll tell you about only one kind but I shall use the 
words ''electron,'' ''proton," "nucleus," "atom," 
and "molecule," about some of which nothing was 
known when I was a boy. 

We used a straight-sided glass jar which would 
hold about a gallon. On the bottom we set a star 
shaped arrangement made of sheets of copper with 
a long wire soldered to it so as to reach up out of the 
jar. Then we poured in a solution of copper sul- 
phate until the jar was about half full. This solution 
was made by dissolving in water crystals of "blue 
vitriol" Avhich we bought at the drug store. 

16 



HOW A BATTERY WORKS 17 

Blue vitriol, or copper sulphate as the chemists 
would call it, is a substance which forms glassy blue 
crystals. Its molecules are formed of copper atoms, 
sulphur atoms, and oxygen atoms. In each molecule 
of it there is one atom of copper, one of sulphur and 
four of oxygen. 

When it dissolves in water the molecules of the 
blue vitriol go wandering out into the spaces between 
the water molecules. But that isn^t all that happens 
or the most important thing for one who is interested 
in making a battery. 

Each molecule is formed by six atoms, that is by 
six little groups of electrons playing about six little 
nuclei. About each nucleus there is going on a game 
but some of the electrons are playing in the game 
about their own nucleus and at the same time taking 
some part in the game which is going on around one 
of the other nuclei. That's why the groups or atoms 
stay together as a molecule. When the molecules 
wander out into the spaces between the water mole- 
cules something happens to this complicated game. 

It will be easiest to see what sort of thing happens 
if we talk about a molecule of ordinary table salt, 
for that has only two atoms in it. One atom is so- 
dium and one is chlorine. The sodium molecule has 
eleven electrons playing around its nucleus. Fairly 
close to the nucleus there are two electrons. Then 
farther away there are eight more and these are hav- 
ing a perfect game. Then still farther away from 
the nucleus there is a single lonely electron. 

The atom of chlorine has seventeen electrons which 



18 LETTERS OF A RADIO-ENGINEER 

play about its nucleus. Close to the nucleus there 
are two. A little farther away there are eight just 
as there are in the sodium atom. Then still farther 
away there are seven. 

TH^LoncLy cL^cTifor^ I am going to draw a pic- 
^^ijcLciy^ ^yj.g ^Y\g. 1) to show what 

I mean, but you must re- 



member that these electrons 



THS: LOnCLV CTLErC 

• • • • 

/iocL£U5 ^ £^c£rcr/?orf are not all in the same plane 
<fftr7£: as if they lay on a sheet of 

^TOM /^rota paper, but are scattered all 

>^^- / around just as they would 

be if they were specks on a ball. 

You see that the sodium atom has one lonely elec- 
tron which hasn't any play fellows and that the 
chlorine atom has seven in its outside circle. It 
appears that eight would make a much better game. 
Suppose that extra electron in the sodium atom goes 
over and plays with those in the chlorine atom so 
as to make eight in the out- 
side group as I have shown • * * . *, * • . 
Fig. 2. That will be all right 

as long as it doesn't get out 

^ . , , „ ., , erne tL£CTnon 

of sight of its own nucleus ^^^-^Sl^^^^^ 

because you remember that the 'c%^'o/^r/er^ro^s /jrfo 

sodium nucleus is responsible ^^%'^rJ'oLicuu^ 

for eleven electrons. The /7<?. ^ 

lonely electron of the sodium atom needn't be lonely 

any more if it can persuade its nucleus to stay so 

close to the chlorine atom that it can play in the 

outer circle of the chlorine atom. 



HOW A BATTERY WORKS 19 

The outer circle of the chlorine atom will then have 
a better game, for it will have just the eight that 
makes a perfect game. This can happen if the 
chlorine atom will stay close enough to the sodium 
atom so that the outermost electron of the sodium 
atom can play in the chlorine circle. You see every- 
thing will be satisfactory if an electron can be 
shared by the two atoms. That can happen only if 
the two atoms stay together; that is, if they form 
a molecule. That's why there are molecules and 
that's what I meant when I spoke of the molecule 
as a big game played by the electrons of two or more 
atoms. 

This molecule which is formed by a sodium atom 
and a chlorine atom is called a molecule of sodium 
chloride by chemists and a molecule of salt by most 
every one who eats it. Something strange happens 
when it dissolves. It wanders around between the 
water molecules and for some reason or other — we 
don't know exactly why — it decides to split up again 
into sodium and chlorine but it can't quite do it. 
The electron which joined the game about the chlor- 
ine nucleus won't leave it. The result is that the 
nucleus of the sodium atom gets away but it leaves 
this one electron behind. 

What gets away isn't a sodium atom for it has 
one too few electrons; and what remains behind 
isn't a chlorine atom for it has one too many elec- 
trons. We call these new groups *4ons" from a 
Greek word which means ^^to go" for they do go, 
wandering off into the spaces between the water 



20 LETTERS OF A RADIO-ENGINEER 

molecules. Fig. 3 gives you an idea of what 
happens. 

You remember that in an atom there are always 
just as many protons as electrons. In this sodium 
ion which is formed when the c^<>^^,^^^§ ^^C«. 
nucleus of the sodium atom <=>^^ ^sr^c^ct^^o^ 
breaks away but leaves behind • • ' • • • 
one planetary electron, there * , 
is then one more proton than t^^^ ao/^^orrso 

there are electrons. Because ^or /7£: ru/^/y^^? 
it has an extra proton, which ^^g^'^ ^^/%7i^^ 
hasn't any electron to asso- yvrf/^r H/Rf^c-n,s irvner^ 
ciate with, we call it a plus iSf/^^^f^T^^'/i; 
ion or a '^positive lon.^' Simi- /7^. s^ 
larly we call the chlorine ion, which has one less 
proton than it has electrons, a minus or ^^ negative 



Now, despite the fact that these ions broke away 
from each other they aren't really satisfied. Any 
time that the sodium ion can find an electron to take 
the place of the one it lost it will welcome it. That 
is, the sodium ion will want to go toward places 
where there are extra electrons. In the same way 
the chlorine ion will go toward places where electrons 
are wanted as if it could satisfy its guilty conscience 
by giving up the electron which it stole from the 
sodium atom, or at least by giving away some other 
electron, for they are all alike anyway. 

Sometimes a positive sodium ion and a negative 
chlorine ion meet in their wanderings in the solu- 
tion and both get satisfied by forming a molecule 



HOW A BATTERY WORKS 21 

again. Even so they don't stay together long before 
they split apart and start wandering again. That's 
what goes on over and over again, millions of times, 
when you dissolve a little salt in a glass of water. 

Now we can see what happens when copper sul- 
phate dissolves. The copper atom has twenty-nine 
electrons about its nucleus and all except two of these 
are nicely grouped for playing their games about 
the nucleus. Two of the electrons are rather out 
of the game, and are unsatisfied. They play with the 
electrons of the part of the molecule which is called 
'^sulphate," that is, the part formed by the sulphur 
atom and the four oxygen atoms. These five atoms 
of the sulphate part stay together very well and 
so we treat them as a group. 

The sulphate group and the copper atom stay to- 
gether as long as they are not in solution but when 
they are, they act very much like the sodium and 
chlorine which I just described. The molecule splits 
up into two ions, one positive and one negative. The 
positive ion is the copper part except that two of the 
electrons which really belong to a copper atom got 
left behind because the sulphate part wouldn't give 
them up. The rest of the molecule is the negative ion. 

The copper ion is a copper atom which has lost two 
electrons. The sulphate ion is a combination of one 
sulphur atom, four oxygen atoms and two electrons 
which it stole from the copper atom. Just as the 
sodium ion is unsatisfied because in it there is one 
more proton than there are electrons, so the copper 
ion is unsatisfied. As a matter of fact it is twice 



22 LETTERS OF A RADIO-ENGINEER 

as badly unsatisfied. It has two more protons than 
it has electrons. We say it has twice the ''electrical 
charge" of the sodium ion. 

Just like a sodium ion the copper ion will tend to 
go toward any place where there are extra electrons 
which it can get to satisfy its own needs. In much 
the same way the sulphate ion will go toward places 
where it can give up its two extra electrons. Some- 
times, of course, as ions of these two kinds wander 
about between the water molecules, they meet and 
satisfy each other by forming a molecule of copper 
sulphate. But if they do they will split apart later 
on; that is, they will ''dissociate" as we should say. 

Now let's go on with the kind of batteries I used 
to make as a boy. You can see that in the solution 
of copper sulphate at the bottom of the jar there was 
always present a lot of positive copper ions and of 
negative sulphate ions. 

On top of this solution of copper sulphate I poured 
very carefully a weak solution of sulphuric acid. 
As I told you, an acid always has hydrogen in its 
molecules. Sulphuric acid has molecules formed by 
two hydrogen atoms and one of the groups which 
we decided to call sulphate. A better name for this 
acid would be hydrogen sulphate for that would im- 
ply that its molecule is the same as one of copper 
sulphate, except that the place of the copper is taken 
by two atoms of hydrogen. It takes two atoms of 
hydrogen because the copper atom has two lonely 
electrons while a hydrogen atom only has one. It 
takes two electrons to fill up the game which the 



HOW A BATTERY WORKS 23 

electrons of the sulphate group are playing. If it 
can get these from a single atom, all right; but if it 
has to get one from each of two atoms, it will do it 
that way. 

I remember when I mixed the sulphuric acid with 
water that I learned to pour the acid into the water 
and not the other way around. Spatterings of sul- 
phuric acid are not good for hands or clothes. With 
this solution I filled the jar almost to the top and 
then hung over the edge a sort of a crow's foot 
shape of cast zinc. The zinc reached down into the 
sulphuric acid solution. There was a binding post 
on it to which a wire could be connected. This wire 
and the one which came from the plate of copper 
at the bottom were the two terminals of the bat- 
tery. We called the wire from the copper ^* posi- 
tive '^ and the one from the zinc ^^ negative.'' 

Now we shall see why and how the battery worked. 
The molecules of sulphuric acid dissociate in solu- 
tion just as do those of copper sulphate. WTien 
sulphuric acid molecules split, the sulphate part goes 
away with two electrons which don't belong to it and 
each of the hydrogen atoms goes away by itself but 
without its electron. We call each a ^^ hydrogen 
ion" but you can see that each is a single proton. 

In the two solutions are pieces of zinc and copper. 
Zinc is like all the rest of the metals in one way. 
Atoms of metals always have lonely electrons for 
which there doesn't seem to be room in the game 
which is going on around their nuclei. Copper as we 
saw has two lonely electrons in each atom. Zinc 



24 LETTERS OF A RADIO-ENGINEER 

also has two. Some metals have one and some two 
and some even more lonely electrons in each atom. 

What happens then is this. The sulphate ions 
wandering around in the weak solution of sulphuric 
acid come along beside the zinc plate and beckon to 
its atoms. The sulphate ions had a great deal rather 
play the game called ' ' zinc sulphate ' ' than the game 
called '' hydrogen sulphate." So the zinc atoms 
leave their places to join with the sulphate ions. 
But wait a minute ! The sulphate ions have two 
extra electrons which they kept from the hydrogen 
atoms. They don't need the two lonely electrons 
which each zinc atom could bring and so the zinc 
atom leaves behind it these unnecessary electrons. 

Every time a zinc atom leaves the plate it fails 
to take all its electrons with it. What leaves the zinc 
plate, therefore, to go into solution is really not a 
zinc atom but is a zinc ion ; that is, it is the nucleus 
of a zinc atom and all except two of the planetary 
electrons. 

Every time a zinc ion leaves the plate there are 
left behind two electrons. The plate doesn't want 
them for all the rest of its atoms have just the same 
number of protons as of electrons. Where are they 
to go 1 We shall see in a minute. 

Sometimes the zinc ions which have got into solu- 
tion meet with sulphate ions and form zinc sulphate 
molecules. But if they do these molecules split up 
sooner or later into ions again. In the upper part of 
the liquid in the jar, therefore, there are sulphate 



HOW A BATTERY WORKS 25 

ions which are negative and two kinds of positive 
ions, namely, the hydrogen ions and the zinc ions. 

Before the zinc ions began to crowd in there were 
just enough hydrogen ions to go with the sulphate 
ions. As it is, the entrance of the zinc ions has in- 
creased the number of positive ions and now there 
are too many. Some of the positive ions, therefore, 
and particularly the hydrogen ions, because the sul- 
phate prefers to associate with the zinc ions, can't 
find enough playfellows and so go down in the jar. 

Down in the bottom of the jar the hydrogen ions 
find more sulphate ions to play with, but that leaves 
the copper ions which used to play with these sul- 
phate ions without any playmates. So the copper 
ions go still further down and join with the copper 
atoms of the copper plate. They haven't much right 
to do so, for you remember that they haven't their 
proper number of electrons. Each copper ion lacks 
two electrons of being a copper atom. Nevertheless 
they join the copper plate. The result is a plate of 
copper which has too few electrons. It needs two 
electrons for every copper ion which joins it. 

How about the zinc plate! You remember that 
it has two electrons more than it needs for every 
zinc ion which has left it. If only the extra electrons 
on the negative zinc plate could get around to the 
positive copper plate. They can if Ave connect a 
wire from one plate to the other. Then the electrons 
from the zinc stream into the spaces between the 
atoms of the wire and push ahead of them the elec- 



26 LETTERS OF A RADIO-ENGINEER 

trons which are wandering around in these spaces. 
At the other end an equal number of electrons leave 
the wire to satisfy the positive copper plate. So we 
have a stream of electrons in the wire, that is, a 
current of electricity and our. battery is working. 

That's the sort of a battery I used to play with. 
If you understand it you can get the general idea of 
all batteries. Let me express it in general terms. 

At the negative plate of a battery ions go into 
solution and electrons are left behind. At the other 
end of the battery positive ions are crowded out 
of solution and join the plate where they cause a 
scarcity of electrons ; that is, make the plate positive. 
If a wire is connected between the two plates, elec- 
trons will stream through it from the negative plate 
to the positive ; and this stream is a current of elec- 
tricity. 





Pl. III. — Dey Battery for Use ix Audiox Circuits (Cour- 
tesy OF National Carbox Co., Ixc). Storage Battery 
(Courtesy of the Electric Storage Battery Co.). 



LETTER 4 



THE BATTERIES IN YOUR RADIO SET 



(This letter may be omitted on the first reading.) 

My Deae Young Man : 

You will need several batteries when you come to 
set up your radio receiver but you won't use such 
clumsy affairs as the gravity cell which I described 
in my last letter. Some of your batteries will be 
dry batteries of the size used in pocket flash lights. 

These are not really dry, for between the plates 
they are filled with a moist paste which is then sealed 
in with wax to keep it from drying out or from spill- 
ing. Instead of zinc and copper these batteries use 
zinc and carbon. No glass jar is needed, for the zinc 
is formed into a jar shape. In this is placed the 
paste and in the center of the paste a rod or bar of 
carbon. The paste doesn't contain sulphuric acid, 
but instead has in it a stuff called sal ammoniac ; that 
is, ammonium chloride. 

The battery, however, acts very much like the 
one I described in my last letter. Ions of zinc leave 
the zinc and wander into the moist paste. These 
ions are positive, just as in the case of the gravity 
battery. The result is that the electrons w^hich used 
to associate with a zinc ion to form a zinc atom are 
left in the zinc plate. That makes the zinc negative 

27 



28 LETTERS OF A RADIO-ENGINEER 

for it has more electrons than protons. The zinc 
ions take the place of the positive ions which are 
already in the paste. The positive ions which origi- 
nally belonged with the paste, therefore, move along 
to the carbon rod and there get some electrons. 
Taking electrons away from the carbon leaves it with 
too many protons ; that is, leaves it positive. In the 
little flash light batteries, therefore, you will always 
find that the round carbon rod, which sticks out of 
the center, is positive and the zinc casing is negative. 

The trouble with the battery like the one I used 
to make is that the zinc plate wastes away. Every 
time a zinc ion leaves it that means that the greater 
part of an atom is gone. Then when the two elec- 
trons which were left behind get a chance to start 
along a copper wire toward the positive plate of the 
battery there goes the rest of the atom. After a 
while there is no more zinc plate. It is easy to see 
what has happened. All the zinc has gone into solu- 
tion or been ** eaten away'' as most people say. Dry 
batteries, however, don't stop working because the 
zinc gets used up, but because the active stuff in the 
paste, the ammonium chloride, is changed into some- 
thing else. 

There's another kind of battery which you will 
need to use with your radio set; that is the storage 
battery. Storage batteries can be used over and 
over again if they are charged between times and 
will last for a long time if properly cared for. Then 
too, they can give a large current, that is, a big 
swift-moving stream of electrons. You will need 



THE BATTERIES IN A RADIO SET 29 

that when you wish to heat the filament of the audion 
in your receiving set. 

The English call our storage batteries by the name 
'^accumulators." I don't like that name at all, but 
I don't like our name for them nearly as well as I 
do the name ''reversible batteries." Nobody uses 
this last name because it's too late to change. Never- 
theless a storage battery is reversible, for it will 
work either way at an instant's notice. 

A storage battery is something like a boy's wagon 
on a hill side. It will run down hill but it can be 
pushed up again for another descent. You can use 
it to send a stream of electrons through a wire from 
its negative plate to its positive plate. Then if 
you connect these plates to some other battery or to a 
generator, (that is, a dynamo) you can make a stream 
of electrons go in the other direction. When you 
have done so long enough the battery is charged 
again and ready to discharge. 

I am not going to tell you very much about the 
storage battery but you ought to know a little about 
it if you are to own and run one with your radio set. 
When it is all charged and ready to work, the nega- 
tive plate is a lot of soft spongy lead held in place 
by a frame of harder lead. The positive plate is a 
lead frame with small squares which are filled with 
lead peroxide, as it is called. This is a substance 
with molecules formed of one lead atom and two 
oxygen atoms. Why the chemists call it lead perox- 
ide instead of just lead oxide I'll tell you some other 
time, but not in these letters. 



30 LETTERS OF A RADIO-ENGINEER 

Between the two plates is a wood separator to 
keep pieces of lead from falling down between and 
touching both plates. You know what would happen 
if a piece of metal touched both plates. There would 
be a short circuit, that is, a sort of a short cut across 
lots by which some of the electrons from the negative 
plate could get to the positive plate without going 
along the wires which we want them to travel. 
That's why there are separators. 

The two plates are in a jar of sulphuric acid so- 
lution. The sulphuric acid has molecules which split 
up in solution, as you remember, into hydrogen ions 
and the ions which we called ^^ sulphate.'' In my 
gravity battery the sulphate ions used to coax the 
zinc ions away into the solution. In the storage bat- 
tery on the other hand the sulphate ions can get to 
most of the lead atoms because the lead is so spongy. 
When they do, they form lead sulphate right where 
the lead atoms are. They don't really need whole 
lead atoms, because they have two more electrons 
than they deserve, so there are two extra electrons 
for every molecule of lead sulphate which is formed. 
That's why the spongy lead plate is negative. 

The lead sulphate won't dissolve, so it stays there 
on the plate as a whitish coating. Now see what 
that means. "What are the hydrogen ions going to 
do ? As long as there was sulphuric acid in the water 
there was plenty of sulphate ions for them to asso- 
ciate with as often as they met ; and they would meet 
pretty often. But if the sulphate ions get tied up 



THE BATTERIES IN A RADIO SET 31 

with the lead of the plate there will be too many 
hydrogen ions left in the solution. Now what are 
the hydrogen ions to do f They are going to get as 
far away from each other as they can, for they are 
nothing bnt protons; and protons don't like to asso- 
ciate. They only stayed around in the first place 
because there was always plenty of sulphate ions 
with whom they liked to play. 

When the hydrogen ions try to get away from 
each other they go to the other plate of the battery, 
and there they will get some electrons, if they have 
to steal in their turn. 

I won't try to tell you all that happens at the 
other plate. The hydrogen ions get the electrons 
which they need, but they get something more. They 
get some of the oxygen away from the plate and so 
form molecules of water. You remember that water 
molecules are made of two atoms of hydrogen and 
one of oxygen. Meanwhile, the lead atoms, which 
have lost their oxygen companions, combine with 
some of the sulphate ions which are in that neighbor- 
hood. During the mix-up electrons are carried away 
from the plate and that leaves it positive. 

The result of all this is a little lead sulphate on 
each plate, a negative plate where the spongy lead 
was, and a positive plate where the lead peroxide 
was. 

Notice very carefully that I said ' ' a little lead sul- 
phate on each plate." The sort of thing I have been 
describing doesn't go on very long. If it did the 



32 LETTERS OF A RADIO-ENGINEER 

battery would run down inside itself and then when 
we came to start our automobile we would have to 
get out and crank. 

How long does it go on? Answer another ques- 
tion first. So far we haven't connected any wire 
between the two plates of the battery, and so none 
of the electrons on the negative plate have any way 
of getting around to the positive plate where elec- 
trons are badly needed. Every time a negative sul- 
phate ion combines with the spongy lead of the nega- 
tive plate there are two more electrons added to that 
plate. You know how well electrons like each other. 
Do they let the sulphate ions keep giving that plate 
more electrons'? There is the other question; and 
the answer is that they do not. Every electron that 
is added to that plate makes it just so much harder 
for another sulphate ion to get near enough to do 
business at all. That's why after a few extra elec- 
trons have accumulated on the spongy lead plate the 
actions which I was describing come to a stop. 

Do they ever begin again? They do just as soon 
as there is any reduction in the number of elec- 
trons which are hopping around in the negative 
plate trying to keep out of each other's way. When 
we connect a wire between the plates we let some of 
these extra electrons of the negative plate pass along 
to the positive plate where they will be welcome. 
And the moment a couple of them start off on that 
errand along comes another sulphate ion in the solu- 
tion and lands two more electrons on the plate. 
That's how the battery keeps on discharging. 



THE BATTERIES IN A RADIO SET 33 

We mustn't let it get too much discharged for the 
lead sulphate is not soluble, as I just told you, and it 
will coat up that plate until there isn't much chance 
of getting the process to reverse. That's why we are 
so careful not to let the discharge process go on too 
long before we reverse it and charge. That's why, 
when the car battery has been used pretty hard to 
start the car, I like to run quite a while to let the gen- 
erator charge the battery again. When the battery 
charges, the process reverses and we get spongy lead 
on the negative plate and lead peroxide on the posi- 
tive plate. 

You've learned enough for one day. Write me 
your questions and I'll answer and then go on in my 
next letter to tell how the audion works. You know 
about conduction of electricity in wires; that is, 
about the electron stream, and about batteries which 
can cause the stream. Now you are ready for the 
most wonderful little device known to science : the 
audion. 



LETTER 5 
GETTING ELECTRONS FROM A HEATED WIRE 

Deae Son : 

I was pleased to get your letter and its questions. 
Yes, a proton is a speck of electricity of the kind 
we call positive and an electron is of the kind we call 
negative. You might remember this simple law; 
'^Like kinds of electricity repel, and unlike attract.'' 

The word ion^ is used to describe any atom, or 
part of a molecule which can travel by itself and 
has more or less than its proper number of electrons. 
By proper number of electrons I mean proper for the 
number of protons which it has. If an ion has more 
electrons than protons it is negative ; if the inequal- 
ity is the other way around it is positive. An atom 
or molecule has neither more nor less protons than 
electrons. It is neutral or ^'uncharged," as we say. 

No, not every substance which will dissolve will 
dissociate or split up into positive and negative ions. 
The salt which you eat will, but the sugar will not. 
If you want a name for those substances which will 
dissociate in solution, call them ^^electrolytes." To 
make a battery we must always use an electrolyte. 

Yes, it is hard to think of a smooth piece of metal 
or a wire as full of holes. Even in the densest solids 
like lead the atoms are quite far apart and there are 

1 If the reader has omitted Letters 3 and 4 he should omit 
this paragraph and the next. 

34 



GETTING ELECTRONS FROM A WIRE 35 

large spaces between the nuclei and the planetary 
electrons of each atom. 

I hope this clears np the questions in your mind 
for I want to get along to the vacuum tube. By a 
vacuum we mean a space which has very few atoms 
or molecules in it, just as few as we can possibly get, 
with the best methods of pumping and exhausting. 
For the present let's suppose that we can get all the 
gas molecules, that is, all the air, out of a little glass 
bulb. 

The audion is a glass bulb like an electric light 
bulb which has in it a thread, or filament, of metal. 
The ends of this filament extend out through the 
glass so that we may connect a battery to them and 
pass a current of electricity through the wire. If we 
do so the wire gets hot. 

What do we mean when we say *Hhe wire gets 
hot!" We mean that it feels hot. It heats the glass 
bulb and we can feel it. But what do we mean in 
words of electrons and atoms'? To answer this we 
must start back a little way. 

In every bit of matter in our world the atoms and 
molecules are in very rapid motion. In gases they 
can move anywhere; and do. That's why odors 
travel so fast. In liquids most of the molecules or 
atoms have to do their moving without getting out 
of the dish or above the surface. Not all of them 
stay in, however, for some are always getting away 
from the liquid and going out into the air above. 
That is why a dish of water will dry up so quickly. 
The faster the molecules are going the better chance 



36 LETTERS OF A RADIO-ENGINEER 

they have of jumping clear away from' the water like 
fish jumping in the lake at sundown. Heating the 
liquid makes its molecules move faster and so more 
of them are able to jump clear of the rest of the 
liquid. That's why when we come in wet we hang 
our clothes where they will get warm. The water 
in them evaporates more quickly when it is heated 
because all we mean by ''heating'' is speeding up 
the molecules. 

In a solid body the molecules can't get very far 
away from where they start but they keep moving 
back and forth and around and around. The hotter 
the body is, the faster are its molecules moving. 
Generally they move a little farther when the body 
is hot than when it is cold. That means they must 
have a little more room and that is why a body is 
larger when hot than when cold. It expands with 
heating because its molecules are moving more rap- 
idly and slightly farther. 

When a wire is heated its molecules and atoms are 
hurried up and they dash back and forth faster than 
before. Now you know that a ^vire, like the filament 
of a lamp, gets hot when the ''electricity is turned 
on," that is, when there is a stream of electrons 
passing through it. Why does it get hot? Because 
when the electrons stream through it they bump and 
jostle their way along like rude boys on a crowded 
sidewalk. The atoms have to step a bit more lively 
to keep out of the way. These more rapid motions 
of the atoms we recognize by the wire growing hot- 
ter. 



GETTING ELECTRONS FROM A WIRE 37 

That is why an electric current heats a wire 
through which it is flowing. Now what happens to 
the electrons, the rude boys who are dodging their 
way along the sidewalk! Some of them are going so 
fast and so carelessly that they will have to dodge 
out into the gutter and off the sidewalk entirely. 
The more boys that are rushing along and the faster 
they are going the more of them will be turned aside 
and plunge off the sidewalks. 

The greater and faster the stream of electrons, 
that is the more current which is flowing through 
the wire, the more electrons will be ^ ' emitted, ' ' that 
is, thrown out of the wire. If you could watch them 
you would see them shooting out of the wire, here, 
there, and all along its length, and going in every 
direction. The number which shoot out each second 
isn't very large until they have stirred things up 
so that the wire is just about red hot. 

What becomes of them? Sometimes they don't get 
very far away from the wire and so come back inside 
again. They scoot off the sidewalk and on again 
just as boys do in dodging their way along. Some 
of them start away as if they were going for good. 

If the wire is in a vacuum tube, as it is in the case 
of the audion, they can't get very far away. Of 
course there is lots of room; but they are going so 
fast that they need more room just as older boys 
who run fast need a larger play ground than do the 
little tots. By and by there gets to be so many of 
them outside that they have to dodge each other 
and some of them are always dodging back into the 



38 LETTERS OF A RADIO-ENGINEER 

wire while new electrons are shooting out from it. 

When there are just as many electrons dodging 
back into the wire each second as are being emitted 
from it the vacuum in the tube has all the electrons 
it can hold. We might say it is '^saturated'' with 
electrons, which means, in slang, ^^full up.'' If any 
more electrons are to get out of the filament just as 
many others which are already outside have to go 
back inside. Or else they have got to be taken away 
somewhere else. 

What I have just told you about electrons getting 
away from a heated wire is very much like what 
happens when a liquid is heated. The molecules of 
the liquid get away from the surface. If we cover 
a dish of liquid which is being heated the liquid mole- 
cules can't get far away and very soon the space 
between the surface of the liquid and the cover gets 
saturated with them. Then every time another mole- 
cule escapes from the surface of the liquid there 
must be some molecule which goes back into the 
liquid. There is then just as much condensation back 
into liquid as there is evaporation from it. That's 
why in cooking they put covers over the vessels when 
they don't want the liquid all to *'boil away." 

Sometimes we sioeak of the vacuum tube in the 
same words we would use in describing evaporation 
of a liquid. The molecules of the liquid which have 
escaped form what is called a '^ vapor" of the liquid. 
As you know there is usually considerable water 
vapor in the air. We say then that electrons are 



GETTING ELECTRONS FROM A WIRE 39 

^^ boiled out" of the filament and that there is a 
^' vapor of electrons" in the tube. 

That is enough for this letter. Next time I shall 
tell you how use is made of these electrons which 
have been boiled out and are free in the space around 
the filament. 



LETTEE 6 



THE AUDION 

Deae Son : 

In my last letter I told how electrons are boiled 
out of a heated filament. The hotter the filament the 
more electrons are emit- 
ted each second. If the 
temperature is kept 
steady, or constant as 
we say, then there are 
emitted each second just 
the same number of elec- 
trons. When the fila- 
ment is enclosed in a 
vessel or glass bulb these 
electrons which get free 
from it cannot go very 
far away. Some of them, 
therefore, have to come 
back to the filament and 
the number which re- 
turns each second is 
just equal to the number 
which is leaving. You 
realize that this is what 
is happening inside an ordinary electric light bulb 
when its filament is being heated. 

40 




r/cp a 



THE AUDION 



41 



An ordinary electric light bulb, however, is not 
an audion although it is like one in the emission of 
electrons from its filament. That reminds me that 
last night as I was waiting for a train I picked up one 
of the Radio Supplements which so many news- 
papers are now running. There was a column of 
enquiries. One letter told how its writer had tried to 
use an ordinary electric light bulb to receive radio 
signals. 

He had plenty of electrons in it but no way to 
control them and make their mo- 
tions useful. In an audion besides 
the filament there are two other 
things. One is a little sheet or 
plate of metal with a connecting 
wire leading out through the glass 
walls and the other is a little wire 
screen shaped like a gridiron and 
r/0O gQ called a ^^grid.'^ It also has a 

connecting wire leading through the glass. Fig. 4 
shows an audion. It will be most convenient, how- 
ever, to represent an audion as in Fig. 5. There you 
see the filament, F^ with its two terminals brought 
out from the tube, the plate, P, and between these 
the grid, G. 

These three parts of the tube are sometimes called 
^'elements.'' Usually, however, they are called 
^'electrodes" and that is why the audion is spoken 
of as the ''three-electrode vacuum tube." An elec- 
trode is what we call any piece of metal or wire 
which is so placed as to let us get at electrons (or 




42 



LETTERS OF A RADIO-ENGINEER 



ions) to control their motions. Let ns see how it 
does so. 

To start ^yith, we shall forget the grid and think of 
a tube ^dth only a filament and a plate in it — a two- 
electrode tube. We shall represent it as in Fig. 6 
and show the battery which heats the filament by 
some lines as at A. In this way of representing a 
battery each cell is represented by a short heavy line 
and a longer lighter line. The hea^^ line stands for 
the negative plate and the longer 
line for the positive plate. We 
shall call the battery which heats 
the filament the ^'filament bat- 
tery'^ or sometimes the ''A-bat- 
tery." As you see, it is formed 
by several battery cells connected 
in series. 

Sometime later I may tell you 
how to connect battery cells to- 
gether and why. For the present all you need to re- 
member is that two batteries are in series if the 
positive plate of one is connected to the negative 
plate of the other. Lf the batteries are alike they 
will pull an electron just twice as hard as either 
could alone. 

To heat the filament of an audion, such as you 
will probably use in your set, will require three stor- 
age-battery cells, like the one I described in my 
fourth letter, all connected in series. We generally 
use storage batteries of about the same size as tliose 
in the automobile. If you will look at the automobile 




/7(?5 




Pl. IV. — Radioteon (Courtesy of Radio Corporation of 
America ) . 



THE AUDION 43 

battery you will see that it is made of three cells 
connected in series. That battery would do very well 
for the filament circuit. 

By the way, do you know what a ^^ circuit" isl 
The word comes from the same Latin word as our 
word ^'circus.'' The Romans were very fond of 
chariot racing at their circuses and built race tracks 
around which the chariots could go. A circuit, there- 
fore, is a path or track around which something can 
race ; and an electrical circuit is a path around which 
electrons can race. The filament, the A-battery and 
the connecting wires of Fig. 6 form a circuit. 

Let us imagine another battery 
formed by several cells in series 
which we shall connect to the tube 
as in Fig. 7. All the positive and 
|j||,i.(,L|,M negative terminals of these bat- 
V -' B -f- teries are connected in pairs, the 
/^/^ 7 positive of one cell to the negative 

of the next, except for one positive and one negative. 
The remaining positive terminal is the positive ter- 
minal of the battery which we are making by this 
series connection. We then connect this positive 
terminal to the plate and the negative terminal to 
the filament as shown in the figure. This new bat- 
tery we shall call the * 'plate battery'^ or the ^'B- 
battery. " 

Now what's going to happen? The B-battery will 
want to take in electrons at its positive terminal 
and to send them out at its negative terminal. The 
positive is connected to the plate in the vacuum tube 




44 LETTERS OF A RADIO-ENGINEER 

of the figure and so draws some of the electrons of 
the plate away from it. Where do these electrons 
come from? They used to belong to the atoms of the 
plate but they were out playing in the space between 
the atoms, so that they came right along when the 
battery called them. That leaves the plate with less 
than its proper number of electrons; that is, leaves 
it positive. So the plate immediately draws to itself 
some of the electrons which are dodging about in 
the vacuum around it. 

Do you remember what was happening in the 
tube? The filament was steadily going on emitting 
electrons although there were already in the tube 
so many electrons that just as many crowded back 
into the filament each second as the filament sent out. 
The filament was neither gaining nor losing elec- 
trons, although it was busy sending them out and 
welcoming them home again. 

When the B-battery gets to work all this is 
changed. The B-battery attracts electrons to the 
plate and so reduces the crowd in the tube. Then 
there are not as many electrons crowding back into 
the filament as there were before and so the filament 
loses more than it gets back. 

Suppose that, before the B-battery was connected 
to the plate, each tiny length of the filament was 
emitting 1000 electrons each second but was getting 
1000 back each second. There was no net change. 
Now, suppose that the B-battery takes away 100 
of these each second. Then only 900 get back to 
the filament and there is a net loss from the filament 



THE AUDION 45 

of 100. Each second this tiny length of filament 
sends into the vacuum 100 electrons which are taken 
out at the plate. From each little bit of filament 
there is a stream of electrons to the plate. Millions 
of electrons, therefore, stream across from filament 
to plate. That is, there is a current of electricity 
between filament and plate and this current con- 
tinues to flow as long as the A-battery and the B- 
battery do their work. 

The negative terminal of the B-battery is con- 
nected to the filament. Every time this battery pulls 
an electron from the plate its negative terminal 
shoves one out to the filament. You know from my 
third and fourth letters that electrons are carried 
through a battery from its positive to its negative 
terminal. You see, then, that there is the same 
stream of electrons through the B-battery as there 
is through the vacuum between filament and plate. 
This same stream passes also through the wires 
which connect the battery to the tube. The path fol- 
lowed by the stream of electrons includes the wires, 
the vacuum and the battery in series. We call this 
path the ''plate circuit." 

We can connect a telephone receiver, or a current- 
measuring instrument, or any thing we wish which 
will pass a stream of electrons, so as to let this same 
stream of electrons pass through it also. All we have 
to do is to connect the instrument in series with the 
other parts of the plate circuit. I'll show you 
how in a minute, but just now I want you to 
understand that we have a stream of electrons, 




46 LETTERS OF A RADIO-ENGINEER 

for I want to tell you how it may be controlled. 

Suppose we use another battery and connect it be- 
tween the grid and the filament so as to make the 
grid positive. That would mean connecting the posi- 
tive terminal of the battery to the grid and the nega- 
tive to the filament as shown by the C-battery of 
Fig. 8. This figure also shows a current-measuring 
instrument in the plate circuit. 

What effect is this C-battery, or grid-battery, go- 
ing to have on the current in the 
plate circuits Making the grid 
positive makes it want electrons. 
It will therefore act just as we 
saw that the plate did and pull Djl^idTL^NiiiiiiiiNi 
electrons across the vacuum to- c - ^ h- *- a + 
wards itself. Fl Q <3 

What happens then is something like this : Elec- 
trons are freed at the filament; the plate and the 
grid both call them and they start off in a rush. 
Some of them are stopped by the wires of the grid 
but most of them go on by to the plate. The grid is 
mostly open space, you know, and the electrons move 
as fast as lightning. They are going too fast in 
the general direction of the grid to stop and look for 
its few and small wires. 

When the grid is positive the grid helps the plate 
to call electrons away from the filament. Mak- 
ing the grid positive, therefore, increases the stream 
of electrons between filament and plate; that is, in- 
creases the current in the plate circuit. 

We could get the same effect so far as concerns 




THE AUDION 47 

an increased plate current by using more batteries in 
series in the plate circuit so as to pull harder. But 
the grid is so close to the filament that a single bat- 
tery cell in the grid circuit can call electrons so 
strongly that it would take several extra battery 
cells in the plate circuit to produce the same effect. 
If we reverse the grid battery, 
as in Fig. 9, so as to make the 
grid negative, then, instead of at- 
tracting electrons the grid repels 
LjbLlthem. Nowhere near as many 
/7+ - B + electrons will stream across to 
FIg 3 the plate when the grid says, 

^*No, go back.'' The grid is in a strategic position 
and what it says has a great effect. 

When there is no battery connected to the grid 
it has no possibility of influencing the electron 
stream which the plate is attracting to itself. We 
say, then, that the grid is uncharged or is at ^^zero 
potential," meaning that it is zero or nothing in pos- 
sibility. But when the grid is charged, no matter 
how little, it makes a change in the plate current. 
When the grid says *^Come on," even though very 
softly, it has as much effect on the electrons as if 
the plate shouted at them, and a lot of extra elec- 
trons rush for the plate. But when the grid whis- 
pers *^Go back," many electrons which would other- 
wise have gone streaking off to the plate crowd back 
toward the filament. That's how the audion works. 
There is an electron stream and a wonderfully sensi- 
tive way of controlling the stream. 



LETTEE 7 
HOW TO MEASURE AN ELECTRON STREAM 

(This letter may be omitted on the first reading.) 

Deak Youth : 

If we are to talk about the audion and how its 
grid controls the current in the plate circuit we must 
know something of how to measure currents. An 
electric current is a stream of electrons. We meas- 
ure it by finding the rate at which electrons are 
traveling along through the circuit. 

What do we mean by the word ^ ' rate ! ' ' You know 
what it means when a speedometer says twenty miles 
an hour. If the car should keep going just as it was 
doing at the instant you looked at the speedometer it 
would go twenty miles in the next hour. Its rate is 
twenty miles an hour even though it runs into a 
smash the next minute and never goes anywhere 
again except to the junk heap. 

It's the same when we talk of electric currents. 
We say there is a current of such and such a number 
of electrons a second going by each point in the cir- 
cuit. We don't mean that the current isn't going 
to change, for it may get larger or smaller, but we 
do mean that if the stream of electrons keeps going 
just as it is there will be such and such a number 
of electrons pass by in the next second. 

In most of the electrical circuits with which you 

48 



MEASURING ELECTRON STREAMS 49 

will deal you will find that electrons must be pass- 
ing along in the circuit at a most amazing rate if 
there is to be any appreciable effect. "When you 
turn on the 40- watt light at your desk you start them 
going through the filament of the lamp at the rate of 
about two and a half billion billion each second. You 
have stood on the sidewalk in the city and watched the 
people stream past you. Just suppose you could 
stand beside that narrow little sidewalk which the 
filament offers to the electrons and count them as 
they go by. We don't try to count them although 
we do to-day know about how many go by in a second 
if the current is steady. 

If some one asks you how old you are you don't say 
^^ About ^ye hundred million seconds"; you tell him 
in years. When some one asks how large a current 
is flowing in a wire we don't tell him six billion billion 
electrons each second; we tell him *^one ampere." 
Just as we use years as the units in which to count 
up time so we use amperes as the units in which 
to count up streams of electrons. When a wire is 
carrying a current of one ampere the electrons are 
streaming through it at the rate of about 6,000,000,- 
000,000,000,000 a second. 

Don't try to remember this number but do remem- 
ber that an ampere is a unit in which we measure cur- 
rents just as a year is a unit in which we measure 
time. An ampere is a unit in which we measure 
streams of electrons just as ^' miles per hour" is a 
unit in which we measure the speed of trains or auto- 
mobiles. 



50 LETTERS OF A RADIO-ENGINEER 

If you wanted to find the weight of something you 
would take a scale and weigh it, wouldn 't you ! You 
might take that spring balance which hangs out in 
the kitchen. But if the spring balance said the 
thing weighed five pounds how would you know if it 
was right? Of course you might take what ever 
it was down town and weigh it on some other scales 
but how would you know those scales gave correct 
weight 1 

The only way to find out would be to try the 
scales with weights which you were sure were right 
and see if the readings on the scale correspond 
to the kno\vn weights. Then you could trust it to 
tell you the weight of something else. That's the 
way scales are tested. In fact that's the way that 
the makers know how to mark them in the first place. 
They put on known weights and marked the lines and 
figures which you see. What they did was called 
** calibrating" the scale. You could make a scale 
for yourself if you mshed, but if it was to be reliable 
you would have to find the places for the markings 
by applying kno^m weights, that is, by calibration. 

How would you know that the weights you used 
to calibrate your scale were really what you thought 
them to be? You would have to find som.e place 
where they had a weight that everybody would agree 
was correct and then compare your weight with that. 
You might, for example, send your pound weight to 
the Bureau of Standards in Washington and for a 
small payment have the Bureau compare it with the 
pound which it keeps as a standard. 



■I 



MEASURING ELECTRON STREAMS 51 

That is easy where one is interested in a pound. 
But it is a little different when one is interested in 
an ampere. You can't make an ampere out of a 
piece of platinum as you can a standard pound 
weight. An ampere is a stream of electrons at about 
the rate of six billion billion a second. No one could 
ever count anywhere near that many, and yet every- 
body who is concerned with electricity wants to be 
able to measure currents in amperes. How is it 
done? 

First there is made an instrument which will have 
something in it to move when electrons are flowing 
through the instrument. We want a meter for the 
flow of electrons. In the basement we have a meter 
for the flow of gas and another for the flow of water. 
Each of these has some part which will move when 
the water or the gas passes through. But they are 
both arranged with little gear wheels so as to keep 
track of all the water or gas which has flowed 
through; they won't tell the rate at which the gas or 
water is flowing. They are like the odometer on the 
car which gives the **trip mileage'' or the ^Hotal 
mileage." We want a meter like the speedometer 
which will indicate at each instant just how fast the 
electrons are streaming through it. 

There are several kinds of meters but I shall not 
try to tell you now of more than one. The simplest 
to understand is called a ^^ hot-wire meter." You 
already know that an electron stream heats a wire. 
Suppose a piece of fine wire is fastened at the two 
ends and that there are binding posts also fastened 



52 LETTERS OF A RADIO-ENGINEER 

to these ends of the wire so that the wire may be 
made part of the circuit where we want to know 
the electron stream. Then the same stream of elec- 
trons will flow through the fine wire as through the 
other parts of the circuit. Because the wire is fine it 
acts like a very narrow sidewalk for the stream 
of electrons and they have to bump and jostle pretty 
hard to get through. That's why the wire gets 
heated. 

You know that a heated wire expands. This wire 
expands. It grows longer and be- 
cause it is held firmly at the ends 
it must bow out at the center. The 
bigger the rate of flow of electrons 
the hotter it gets; and the hotter 
it gets the more it bows out. At _/^ ^^ f^Jl^^^ 
the center we might fasten one end 
— the short end — of a little lever. o^^THSr%o7^Jvf/^£: 
A small motion of this short end F7 Q / O 
of the lever will mean a large motion of the other 
end, just like a ^'teeter board'' when one end is 
longer than the other; the child on the long end 
travels further than the child on the short end. The 
lever magnifies the motion of the center of the hot 
wire part of our meter so that we can see it easier. 

There are several ways to make such a meter. 
The one shown in Fig. 10 is as easy to understand 
as any. We shape the long end of the lever like a 
pointer. Then the hotter the wire the farther the 
pointer moves. 

If we could put this meter in an electric circuit 




MEASURING ELECTRON STREAMS 



53 



X 




JO o- 






where we knew one ampere was flowing we could 
put a numeral ^ ' 1 ' ' opposite where the pointer stood. 
Then if we could increase the current until there were 
two amperes flowing through the meter we could 
mark that position of the pointer ''2'' and so on. 
That 's the way we would calibrate the meter. After 
we had done so we would call it an "ammeter'' be- 
cause it measures amperes. Years ago people would 
have called it an '^amperemeter" but no one who is 
up-to-date would call it so to-day. 

If we had a very carefully 
made ammeter we would 
send it to the Bureau of 
Standards to be calibrated. 
At the Bureau they have a 
number of meters which 
they know are correct in 
their readings. They would 
put one of their meters and 
ours into the same circuit so that both carry the 
same stream of electrons as in Fig. 11. Then 
whatever the reading was on their meter could be 
marked opposite the pointer on ours. 

Now I want to tell you how the physicists at the 
Bureau know what is an ampere. Several years ago 
there was a meeting or congress of physicists and 
electrical engineers from all over the world who dis- 
cussed what they thought should be the unit in which 
to measure current. They decided just what they 
would call an ampere and then all the countries from 
which they came passed laws saying that an ampere 



WHICH l£ 

TO eo 






riq// 






\ 



54 LETTERS OF A RADIO-ENGINEER 

should be what these scientists had recommended. 
To-day, therefore, an ampere is defined by law. 

To tell when an ampere of current is flowing re- 
quires the use of two silver plates and a solution 
of silver nitrate. Silver nitrate has molecules made 
up of one atom of silver combined with a group of 
atoms called "nitrate." You remember that the mole- 
cule of copper sulphate, discussed in our third letter, 
was formed by a copper atom and a group called sul- 
phate. Nitrate is another group something like sul- 
phate for it has oxygen atoms in it, but it has three 
instead of four, and instead of a sulphur atom there 
is an atom of nitrogen. 

When silver nitrate molecules go into solution they 
break up into ions just as copper sulphate does. One 
ion is a silver atom which has lost one electron. This 
electron was stolen from it by the nitrate part of the 
molecule when they dissociated. The nitrate ion, 
therefore, is formed by a nitrogen atom, three oxy- 
gen atoms, and one extra electron. 

If w^e put two plates of silver into such a solution 
nothing will happen until we connect a battery to 
the plates. Then the battery takes electrons away 
from one plate and gives electrons to the other. 
Some of the atoms in the plate which the battery is 
robbing of electrons are just like the silver ions which 
are moving around in the solution. That's why they 
can go out into the solution and play with the nitrate 
ions each of which has an extra electron which it stole 
from some silver atom. But the moment silver ions 



MEASURING ELECTRON STREAMS 55 

leave their plate we have more silver ions in the 
solution than we do sulphate ions. 

The only thing* that can happen is for some of 
the silver ions to get out of the solution. They 
aren't going back to the positive silver plate from 
which they just came. They go on toward the nega- 
tive plate where the battery is sending an electron 
for every one which it takes away from the positive 
plate. There start off towards the negative plate, 
not only the ions which just came from the positive 
plate, but all the ions that are in the solution. The 
first one to arrive gets an electron but it can't take 
it away from the silver plate. And why should it? 
As soon as it has got this electron it is again a nor- 
mal silver atom. So it stays with the other atoms in 
the silver plate. That's what happens right along. 
For every atom which is lost from the positive plate 
there is one added to the negative plate. The silver 
of the positive plate gradually wastes away and the 
negative plate gradually gets an extra coating of 
silver. 

Every time the battery takes an electron away 
from the positive plate and gives it to the negative 
plate there is added to the negative plate an atom 
of silver. If the negative plate is weighed before 
the battery is connected and again after the battery 
is disconnected we can tell how much silver has been 
added to it. Suppose the current has been perfectly 
steady, that is, the same number of electrons stream- 
ing through the circuit each second. Then if we 



56 LETTERS OF A RADIO-ENGINEER 

know how long the current has been rnnning we can 
tell how much silver has been deposited each second. 

The law says that if silver is being deposited at 
the rate of 0,001118 gram each second then the cur- 
rent is one ampere. That's a small amount of silver, 
only about a thousandth part of a gram, and you 
know that it takes 28.35 grams to make an ounce. 
It's a very small amount of silver but it's an enor- 
mous number of aton>s. How many? Six billion 
billion, of course, for there is deposited one atom for 
each electron in the stream. 

In my next letter I'll tell you how we measure the 
pull which batteries can give to electrons, and then 
we shall be ready to go on with more about the 
audion. 



LETTER 8 
ELECTRON-MOVING-FORCES 

(This letter may be omitted on the first reading.) 

Deae Young Man : 

I trust you have a fairly good idea that an ampere 
means a stream of electrons at a certain definite rate 
and hence that a current of say 3 amperes means a 
stream with three times as many electrons passing 
along each second. 

In the third and fourth letters you found out why a 
battery drives electrons around a conducting circuit. 
You also found that there are several different kinds 
of batteries. Batteries differ in their abilities to 
drive electrons and it is therefore convenient to 
have some way of comparing them. "We do this by 
measuring the electron-moving-force or ^^electro- 
motive force ^' which each battery can exert. To 
express electromotive force and give the results of 
our measurements we must have some unit. The 
unit we use is called the ^^volt." 

The volt is defined by law and is based on the 
suggestions of the same body of scientists who rec- 
ommended the ampere of our last letter. They de- 
fined it by telling how to make a particular kind 
of battery and then saying that this battery had an 
electromotive force of a certain number of volts. 
One can buy such standard batteries, or standard 

57 



58 LETTERS OF A RADIO-ENGINEER 

cells as they are called, or he can make them for him- 
self. To be sure that they are just right he can then 
send them to the Bureau of Standards and have 
them compared with the standard cells which the 
Bureau has. 

I don't propose to tell you much about standard 

cells for you won't have to use them until you come 

to study physics in real earnest. They are not good 

for ordinary purposes because the moment they go 

to work driving electrons the conditions inside them 

change so their electromotive force is 

changed. They are delicate little affairs 

and are useful only as standards with 

I — -JH|-^ — ' which to compare other batteries. And 

FlQ /2 even as standard batteries they must be 

used in such a way that they are not 

required to drive any electrons. 

Let's see how it can be done. Suppose two boys 
sit opposite each other on the floor and brace their 
feet together. Then with their hands they take hold 
of a stick and pull in opposite directions. If both 
have the same stick-motive-force the stick w^'ll not 
move. 

Now suppose we connect the negative feet — I mean 
negative terminals — of two batteries together as in 
Fig. 12. Then we connect their positive terminals 
together by a wire. In the wire there will be lots 
of free electrons ready to go to the positive plate of 
the battery which pulls the harder. If the batteries 
are equal in electromotive force these electrons will 
stay right where they are. There will be no stream 



ELECTRON-MOVING-FORCES 



59 




DRY B/=^TTE:Rf^S 




of electrons and yet we'll be using one of the bat- 
teries to compare with the other. 

That is all right, you think, but what are we to 
do when the batteries are not 
just equal in e. m. i,% (e. m. 
f. is code for electromotive 
force). I'll tell you, because 
the telling includes some oth- 
er ideas which will be valua- 
ble in your later reading. 

Suppose we take batteries 
which aren't going to be in- 
jured by being made to work 
— storage batteries will do 
nicely — and connect them in 
series as in Fig. 13. When 
batteries are in series they 
act like a single stronger battery, one whose e. m. f. 
is the sum of the e. m. f . 's of the separate batteries. 
Connect these batteries to a long fine wire as in 
Fig. 14. There is a stream of electrons along this 
wire. Next connect the nega- 
tive terminal of the standard 
cell to the negative terminal of 
the storage batteries, that is, 
brace their feet against each 
other. Then connect a wire to 
the positive terminal of the standard cell. This wire 
acts just like a long arm sticking out from the posi- 
tive plate of this cell. 

Touch the end of the wire, which is ^ of Fig. 14, 






X 






60 LETTERS OF A RADIO-ENGINEER 

to some point as a on the fine wire. Now what do we 
have? Right at a, of course, there are some free elec- 
trons and they hear the calls of both batteries. If the 
standard battery, S of the figure, calls the stronger 
they go to it. In that case move the end p nearer 
the positive plate of the battery B, so that it will 
have a chance to exert a stronger pull. Suppose we 
try at c and find the battery B is there the stronger. 
Then we can move back to some point, say h^ where 
the pulls are equal. 

To make a test like this we put a sensitive current- 
measuring instrument in the wire Avhich leads from 
the positive terminal of the standard cell. We also 
use a long fine wire so that there can never be much 
of an electron stream anyway. When the pulls are 
equal there will be no current through this instru- 
ment. 

As soon as we find out where the proper setting 
is we can replace S By some other battery, say X, 
which we wish to compare with S. We find the set- 
ting for that battery in the same way as we just did 
for S. Suppose it is at d in Fig. 14 while the setting 
for S was at h. We can see at once that X is stronger 
than S. The question, however, is how much 
stronger. 

Perhaps it would be better to try to answer this 
question by talking about e. m. f.'s. It isn't fair to 
speak only of the positive plate which calls, we must 
speak also of the negative plate which is shooing 
electrons away from itself. The idea of e. m. f . takes 
care of both these actions. The steady stream of 



, 



ELECTRON-MOVING-FORCES 61 

electrons in the fine wire is due to the e. m. f. of the 
battery B, that is to the pull of the positive terminal 
and the shove of the negative. 

If the wire is uniform, that is the same through- 
out its length, then each inch of it requires just as 
much e. m. f . as any other inch. Two inches require 
twice the e. m. f . which one inch requires. We know 
how much e. m. f . it takes to keep the electron stream 
going in the part of the wire from n to h. It takes 
just the e. m. f . of the standard cell, 8, because when 
that had its feet braced at n it pulled just as hard at h 
as did the big battery B. 

Suppose the distance n io d (usually written nd) 
is twice as great as that from n io h (nb). That 
means that battery X has twice the e. m. f . of battery 
S, You remember that X could exert the same force 
through the length of wire nd, as could the large bat- 
tery. That is twice what cell S can do. Therefore 
if we know how many volts to call the e. m. f . of the 
standard cell we can say that X has an e. m. f. of 
twice as many volts. 

If we measured dry batteries this way we should 
find that they each had an e. m. f. of about 1.46 volts. 
A storage battery would be found to have about 2.4 
volts when fully charged and perhaps as low as 2.1 
volts when we had run it for a while. 

That is the way in which to compare batteries 
and to measure their e. m. f.'s, but you see it takes 
a lot of time. It is easier to use a ^ ' voltmeter ' ' which 
is an instrument for measuring e. m. f.'s. Here is 
how one could be made. 



62 LETTERS OF A RADIO-ENGINEER 

First there is made a current-measuring instru- 
ment which is quite sensitive, so that its pointer will 
show a deflection when only a very small stream of 
electrons is passing through the instrument. We 
could make one in the same way as we made the 
ammeter of the last letter but there are other bet- 
ter ways of which I'll tell you later. Then we 
connect a good deal of fine wire in series with the 
instrument for a reason which I'll tell you in a min- 
ute. The next and last step is to calibrate. 

We know how many volts of e. m. f . are required to 
keep going the electron stream between n and b — 
we know that from the e. m. f. 



^ 




of our standard cell. Suppose -.L 

then that we connect this new "" 

instrument, which we have just 

made, to the wire at n and h as [ |<g:> 

in Fig. 15. Some of the elec- yoZ^cT^:/^ 

trons at n which are so anxious F'lq /3 

to get away from the negative plate of battery B 

can now travel as far as h through the wire of 

the new instrument. They do so and the pointer 

swings around to some new position. Opposite that 

we mark the number of volts which the standard 

battery told us there was between n and h. 

If we move the end of the wire from h io d the 
pointer will take a new position. Opposite this we 
mark twice the number of volts of the standard cell. 
We can run it to a point e where the distance ne is 
one-half nb, and mark our scale with half the num- 
ber of volts of the standard cell, and so on for other 



ELECTRON-MOVING-FORCES 63 

positions along the wire. That's the way we cali- 
brate a sensitive current-measuring instrument 
(with its added wire, of course) so that it will read 
volts. It is now a voltmeter. 

If we connect a voltmeter to the battery X as in 
Fig. 16 the pointer will tell us the number of volts 
in the e. m. f. of X, for the pointer will take the 
same position as it did when the voltmeter was con- 
nected between n and d. 

There is only one thing to watch out for in all 
this. We must be careful that the voltmeter is so 
made that it won't offer too easy 
a path for electrons to follow. We 



<J^ 



n<^ le 



only want to find how hard a bat- 
tery can pull an electron, for that 
is what we mean by e. m. f . Of 
course, we must let a small stream 
of electrons flow through the voltmeter so as to make 
the pointer move. That is why voltmeters of this 
kind are made out of a long piece of fine wire or else 
have a coil of fine wire in series with the current- 
measuring part. The fine wire makes a long and 
narrow path for the electrons and so there can be 
only a small stream. Usually we describe this con- 
dition by saying that a voltmeter has a high resist- 
ance. 

Fine wires offer more resistance to electron 
streams than do heavy wires of the same length. If 
a wire is the same diameter all along, the longer the 
length of it which we use the greater is the resist- 
ance which is offered to an electron stream. 



64 



LETTERS OF A RADIO-ENGINEER 



You will need to know how to describe the resist- 
ance of a wire or of any part of an electric circuit. 
To do so you tell how many ^^ohms" of resistance 
it has. The ohm is the unit in which we measure the 
resistance of a circuit to an electron stream. 

I can show you what an ohm is if I tell you a 
simple way to measure a resistance. Suppose you 
have a wire or coil of wire and want to know its 
resistance. Connect it in series ^^r^^rs"/^ 
with a battery and an ammeter 
as shown in Fig. 17, The same 
electron stream passes through 
all parts of this circuit and 
the ammeter tells us what this 
stream is in amperes. Now 
connect a voltmeter to the two 
ends of the coil as shown in 
the fisrure. The voltmeter tells 



<J^ 



tL-m 






<3=:> 



in volts how much e. m. f. is being applied to force 
the current through the coil. Divide the number of 
volts by the number of amperes and the quotient 
(answer) is the number of ohms of resistance in 
the coil. 

Suppose the ammeter shows a current of one am- 
pere and the voltmeter an e. m. f. of one volt. Then 
dividing 1 by 1 gives 1. That means that the coil 
has a resistance of one ohm. It also means one ohm 
is such a resistance that one volt will send through 
it a current of one ampere. You can get lots of 
meaning out of this. For example, it means also 



ELECTRON-MOVING-FORCES 65 

that one volt will send a current of one ampere 
through a resistance of one ohm. 

How many ohms would the coil have if it took 5 
volts to send 2 amperes through it. Solution: Di- 
vide 5 by 2 and you get 2.5. Therefore the coil would 
have a resistance of 2.5 ohms. 

Try another. If a coil of resistance three ohms 
is carrying two amperes what is the voltage across 
the terminals of the coil? For 1 ohm it would take 
1 volt to give a current of 1 ampere, wouldn't it? 
For 3 ohms it takes three times as much to give one 
ampere. To give twice this current would take 
twice 3 volts. That is, 2 amperes in 3 ohms re- 
quires 2x3 volts. 

Here 's one for you to try by yourself. If an e. m. f . 
of 8 volts is sending current through a resistance of 2 
ohms, how much current is flowing? Notice that I 
told the number of ohms and the number of volts, 
what are you going to tell? Don't tell just the num- 
ber; tell how many and what. 



LETTER 9 
THE AUDION-CHARACTERISTIC 

My Deae Young Student: 

Although there is much in Letters 7 and 8 which 
it is well to learn and to think about, there are only- 
three of the ideas which you must have firmly- 
grasped to get the most out of this letter which I am 
now going to write you about the audion. 

First : Electric currents are streams of electrons. 
We measure currents in amperes. To measure a 
current we may connect into the circuit an ammeter. 

Second : Electrons move in a circuit when there is 
an electron-moving-force, that is an electromotive 
force or e. m. f. We measure e. m. f.'s in volts. To 
measure an e. m. f. we connect a voltmeter to the two 
points between which the e. m. f. is active. 

Third : What current any particular e. m. f . will 
cause depends upon the circuit in which it is active. 
Circuits differ in the resistance which they offer 
to e. m. f.'s. For any particular e. m. f. (that is for 
any given e. m. f.) the resulting current will be 
smaller the greater the resistance of the circuit. We 
measure resistance in ohms. To measure it we find 
the quotient of the number of volts applied to the 
circuit by the number of amperes which flow. 

In my sixth letter I told you something of how the 
audion works. It would be worth while to read again 
that letter. You remember that the current in the 

66 



THE AUDION-CHARACTERISTIC 



67 



plate circuit can be controlled by the e. m. f . which 
is applied to the grid circuit. There is a relationship 
between the plate current and the grid voltage which 
is peculiar or characteristic to the tube. So we call 
such a relationship ^'a characteristic." Let us see 
how it may be found and what it will be. 

Connect an ammeter in the plate- or B-circuit, of 
the tube so as to measure the plate-circuit current. 
You will find that almost all books use the letter ''/'' 
to stand for current. The reason is that scientists 

used to speak of the 'in- 
tensity of an electric cur- 
rent" so that 'T' really 
stands for intensity. We 
use I to stand for something 
more than the word '^cur- 
rent." It is our symbol for 
/vc? /S whatever an ammeter would 
read, that is for the amount of current. 

Another convenience in symbols is this : We shall 
frequently want to speak of the currents in several 
different circuits. It saves time to use another letter 
along with the letter I to show the circuit to which 
we refer. For example, we are going to talk about 
the current in the B-circuit of the audion, so we 
call that current Ib. We write the letter B below 
the line on which I stands. That is why we say the 
B is subscript, meaning ^^ written below." When 
you are reading to yourself be sure to read Ib as 
^^ eye-bee" or else as ^^ eye-subscript-bee." Ib there- 
fore will stand for the number of amperes in the 




68 LETTERS OF A RADIO-ENGINEER 

plate circuit of the audion. In tlie same way la 
would stand for the current in the filament circuit. 

We are going to talk about e. m. f.'s also. The 
letter "E^' stands for the number of volts of e. m. f. 
in a circuit. In the filament circuit the battery has 
Ea volts. In the plate circuit the e. m. f . is Eb volts. 
If we put a battery in the grid circuit we can let Eo 
represent the number of volts applied to the grid- 
filament or C-circuit. 

The characteristic relation which we are after is 
one between grid voltage, that is Ec, and plate cur- 
rent, that is Ib. So we call it the Ec-Ib characteristic. 
The dash between the letters is not a subtraction 
sign but merely a dash to separate the letters. Now 
we'll find the "ee-see-eye-bee" characteristic. 

Connect some small dry cells in series for use 
in the grid circuit. Then connect the filament to the 
middle cell as in Fig. 19. Take the wire which comes 
from the grid and put a battery clip on it, then you 
can connect the grid anywhere you want along this 
series of batteries. See Fig. 18. In the figure this 
movable clip is represented by an arrow head. You 
can see that if it is at a the battery will make the 
grid positive. If it is moved to h the grid will be 
more positive. On the other hand if the clip is at o 
there will be no e. m. f . applied to the grid. If it is 
at c the grid will be made negative. 

Between grid and filament there is placed a volt- 
meter which will tell how much e. m. f . is applied to 
the grid, that is, tell the value of Ecy for any position 
whatever of the clip. 



THE AUDION-CHARACTERISTIC 



69 



We shall start with the filament heated to a deep 
red. The manufacturers of the audion tell the pur- 
chaser what current should flow through the fila- 
ment so that there will be the proper emission of 
electrons. There are easy ways of finding out for 
one's self but we shall not stop to describe them. The 
makers also tell how many volts to apply to the plate, 
that is what value Eb should have. We could find 



n 






/=7<?/S 




/7W/V£-7i£7? 



I. 



H 



this out also for ourselves but we shall not stop to do 
so. 

Now we set the battery clip so that there is no 
voltage applied to the grid ; that is, we start with Eo 
equal to zero. Then we read the ammeter in the 
plate circuit to find the value of Ib which corresponds 
to this condition of the grid. 

Next we move the clip so as to make the grid as 
positive as one battery will make it, that is we move 
the clip to a in Fig. 19. We now have a different 
value of Ec and will find a different value of Ib when 
we read the ammeter. Next move the clip to apply 
two batteries to the grid. We get a new pair of 
values for Ec and 7b, getting Eo from the voltmeter 
and Ib from the ammeter. As we continue in this 
way, increasing Ec, we find that the current Ib in- 



70 LETTERS OF A RADIO-ENGINEER 

creases for a while and then after we have reached a 
certain value of Eg the current Is stops increasing. 
Adding more batteries and making the grid more 
positive doesn't have any effect on the plate current. 
Before I tell you why this happens I want to show 
you how to make a picture of the pairs of values of 







































\ 














V 


\, 












\ 


\ 














\i 





f=>ostTiVs 2 ^ves 
ZERO /^i/e: 



-/■ 2.$sr 7*-/. 2 ^yc 
the: s/le:ht 

/s>OUiC£:M/=)N 



I I I 



F-zq 20 



Ec and Is which we have been reading on the volt- 
meter and ammeter. 

Imagine a city where all the streets are at right 
angles and the north and south streets are called 
streets and numbered while the east and west thoro- 
fares are called avenues. I'll draw the map as in 
Fig. 20. Right through the center of the city goes 
Main Street. But the people who laid out the roads 
were mathematicians and instead of calling it Main 
Street they called it ''Zero Street." The first street 
east of Zero St. we should have called ''East First 
Street" but they called it "Positive 1 St," and the 



THE AUDION-CHARACTERISTIC 71 

next beyond ^^ Positive 2 St.," and so on. West of 
the main street they called the first street '^Nega- 
tive 1 St." and so on. 

When they came to name the avenues they were 
jnst as precise and mathematical. They called the 
main avenue ''Zero Ave." and those north of it 
"Positive 1 Ave.," "Positive 2 Ave." and so on. 
Of course, the avenues south of Zero Ave. they called 
Negative. ^ 

The Town Council went almost crazy on the sub- 
ject of numbering ; they numbered everything. The 
silent policeman which stood at the corner of "Posi- 
tive 2 St." and "Positive 1 Ave." was marked that 
way. Half way between Positive 2 St. and Positive 
3 St. there was a garage which set back about two- 
tenths of a block from Positive 1 Ave. The Council 
numbered it and called it "Positive 2.5 St. and Pos- 
itive 1.2 Ave." Most of the people spoke of it as 
"Plus 2.5 St. and Plus 1.2 Ave." 

Sometime later there was an election in the city 
and a new Council was elected. The members were 
mostly young electricians and the new Highway 
Commissioner was a radio enthusiast. At the first 
meeting the Council changed the names of all the 
avenues to "Mil-amperes" ^ and of all the streets to 
"Volts." 

Then the Highway Commissioner who had just 
been taking a set of voltmeter and ammeter readings 
on an audion moved that there should be a new 

1 A mil-ampere is a thousandth of an ampere just as a millimeter 
is a thousandth of a meter. 



72 LETTERS OF A RADIO-ENGINEER 

road known as ''Audion Characteristic." He said 
the road should pass through the following points: 
Zero Volt and Plus 1.0 Mil-ampere 
Plus 2.0 Volts and Plus 1.7 Mil-amperes 
Plus 4.0 Volts and Plus 2.6 Mil-amperes 
Plus 6.0 Volts and Plus 3.4 Mil-amperes 
Plus 8.0 Volts and Plus 4.3 Mil-amperes 
And so on. Fig. 21 shows the new road. 



/»OSlTlve /O Ml CLt/^rnP'^RHiS 




^ 


























^ 






Q 






















y 


y 






n 




















/ 


/ 








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/ 


V 












7m 


VE^ 


■A7/ 


Ulfi 


}MP 






t 


^' 












/ 


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/ 


b'^ 
















2 








/ 


V 


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/ 


> 


y 
































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Ml 


LLt^ 


IMR 


E/rt 


^s 


^^ ^ 2 O 2 ^ S a /O /2 /^ /S /a 20 22 





/^-/g 2/ 



One member of the Council jumped up and said 
' ' But what if the grid is made negative % ' ' The Com- 
missioner had forgotten to see what happened so he 
went home to take more readings. 

He shifted the battery clip along, starting at c of 



THE AUDION-CHARACTERISTIC 73 

Fig. 22. At the next meeting of the Council he 
brought in the following list of readings and hence 
of points on his proposed road. 

Minus 1.0 Volts and Plus 0.6 Mil-ampere 



li 


2.0 


ii 


i i 


ii 


0.4 


a 


i i 


6 i 


3.0 


a 


ii 


ii 


0.2 


ii 


ii 


ii 


4.0 


ii 


ii 


ii 


0.1 


ii 


ii 


ii 


5.0 


ii 


ii 


ii 


0.0 


a 


i i 



Then he showed the other members of the Council 
on the map of Fig. 23 how the Audion Characteristic 
would look. 






VOLT 



ng az 




mrn^ 



There was considerable discussion after that and 
it appeared that different designs and makes of au- 
dions would have different characteristic curves. 
They all had the same general form of curve but they 
would pass through different sets of points depend- 
ing upon the design and upon the B-battery volt- 
age. It was several meetings later, however, before 
they found out what effects were due to the form of 
the curve. Right after this they found that they 
could get much better results with their radio sets. 

Now look at the audion characteristic. Making 
the grid positive, that is going on the positive side of 
the zero volts in our map, makes the plate current 



74. 



LETTERS OF A RADIO-ENGINEER 



larger. You remember that I told you in Letter 6 
how the grid, when positive, helped call electrons 
away from the filament and so made a larger stream 
of electrons in the plate circuit. The grid calls elec- 
trons away from the filament. It can't call them 
out of it; they have to come out themselves as 1 
explained to you in the fifth letter. 































■ 
























y 


|X 


























/ 


/ 








k 


















/ 


V 










(5: 
















^i 


V' 












S 












y 


V 


r 














li! 










/ 


^r> 
















.^ 








/ 


V 


9^ 

























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^ 2 O 5 


V Q ( 


s, e to /2 /a /6 le zo aa 



- o y- 



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nq23 



You can see that as we make the grid more and 
more positive, that is, make it call louder and louder, 
a condition will be reached where it v/on't do it any 
good to call any louder, for it will already be get- 
ting all the electrons away from the filament just 
as fast as they are emitted. Making the grid more 
positive after that will not increase the plate current 
any. That's why the characteristic flattens off as 
you see at high values of grid voltage. 

The arrangement which we pictured in Fig. 22 for 



THE AUDION-CHARACTERISTIC 



75 



making changes in the grid voltage is simple but it 
doesn^t let us change the voltage by less than that 
of a single battery cell. I want to show you a way 
which will. You'll find it very useful to know and 
it is easily understood for it is something like the 
arrangement of Fig. 14 in the preceding letter. 

Connect the cells as in Fig. 24 to a fine wire. About 
the middle of this wire connect the filament. As 
before use a clip on the end of the wire from the 






f:/<^ a^ 




i[i|i|i|i(i|i[iH 



grid. If the grid is connected to a in the figure there 
is applied to the grid circuit that part of the e. m. f . 
of the battery which is active in the length of wire 
between o and a. The point a is nearer the positive 
plate of the battery than is the point o. So the grid 
will be positive and the filament negative. 

On the other hand, if the clip is connected at h 
the grid will be negative with respect to the filament. 
We can, therefore, make the grid positive or nega- 
tive depending on which side of o we connect the 
clip. How large the e. m. f . is which will be applied 
to the grid depends, of course, upon how far away 
from the clip is connected. 

Suppose you took the clip in your hand and slid 
it along in contact with the wire, first from o to a 



76 LETTERS OF A RADIO-ENGINEER 

and then back again through o to & and so on back 
and forth. You would be making the grid alternately 
positive and negative, wouldn^t you? That is, you 
would be applying to the grid an e. m. f. which in- 
creases to some positive value and then, decreasing 
to zero, reverses, and increases just as much, only to 
decrease to zero, where it started. If you do this 
over and over again, taking always the same time for 
one round trip of the clip you will be impressing 
on the grid circuit an ^^alternating e. m. f/^ 

What's going to happen in the plate circuit? 
When there is no e. m. f. applied to the grid cir- 
cuit, that is when the grid potential (possibilities) 
is zero, there is a definite current in the plate cir- 
cuit. That current we can find from our character- 
istic of Fig. 23 for it is where the curve crosses Zero 
Volts. As the grid becomes positive the current 
rises above this value. When the grid is made nega- 
tive the current falls below this value. The current, 
Ibj then is made alternately greater and less than the 
current when Ec is zero. 

You might spend a little time thinking over this, 
seeing what happens when an alternating e. m. f. 
is applied to the grid of an audion, for that is going 
to be fundamental to our study of radio. 



LETTER 10 
CONDENSERS AND COILS 

Deak Son : 

In the last letter we learned of an alternating 
e. m. f . The way of producing it, which I described, 
is very crude and I want to tell how to make the 
audion develop an alternating e. m. f. for itself. 
That is what the audion does in the transmitting set 
of a radio telephone. But an audion can't do it all 
alone. It must have associated with it some coils 
and a condenser. You know what I mean by coils 
but you have yet to learn about condensers. 

A condenser is merely a gap in an otherwise con- 
ducting circuit. It's a gap across which electrons 
cannot pass so that if there is an e. m. f. in the 
circuit, electrons will be very plentiful on one side 
of the gap and scarce on the other side. If there are 
to be many electrons waiting beside the gap there 
must be room for them. For that reason we usu- 
ally provide waiting-rooms for the electrons on each 
side of the gap. Metal plates or sheets of tinfoil 
serve nicely for this purpose. Look at Fig. 25. You 
see a battery and a circuit which would be conduct- 
ing except for the gap at C. On each side of the gap 
there is a sheet of metal. The metal sheets may be 
separated by air or mica or paraffined paper. The 

77 




78 LETTERS OF A RADIO-ENGINEER 

combination of gap, plates, and whatever is between, 
provided it is not conducting, is called a condenser. 

Let ns see what happens when we connect a bat- 
tery to a condenser as in the figure. 
The positive terminal of the battery 
calls electrons from one plate of the 
condenser while the negative battery- 
terminal drives electrons away from 
itself toward the other plate of the 
condenser. One plate of the conden- / ^^^^ 
ser, therefore, becomes positive while the other 
plate becomes negative. 

You know that this action of the battery will go 
on until there are so many electrons in the negative 
plate of the condenser that they prevent the battery 
from adding any more electrons to that plate. The 
same thing happens at the other condenser plate. 
The positive terminal of the battery calls electrons 
away from the condenser plate which it is making 
positive until so many electrons have left that the 
protons in the atoms of the plate are calling for 
electrons to stay home just as loudly and effectively 
as the positive battery-terminal is calling them away. 

"When both these conditions are reached^ — and they 
are both reached at the same time — then the battery 
has to stop driving electrons around the circuit. 
The battery has not enough e. m. f. to drive any 
more electrons. Why! Because the condenser has 
now just enough e. m. f. with which to oppose the 
battery. 

It would be well to learn at once the right words 



CONDENSERS AND COILS 79 

to use in describing this action. We say that the 
battery sends a ''charging current'' around its cir- 
cuit and ''charges the condenser" until it has the 
same e. m. f. When the battery is first connected 
to the condenser there is lots of space in the waiting- 
rooms so there is a great rush or surge of electrons 
into one plate and away from the other. Just at 
this first instant the charging current, therefore, is 
large but it decreases rapidly, for the moment elec- 
trons start to pile up on one plate of the condenser 
and to leave the other, an e. m. f. builds up on the 
condenser. This e. m. f., of course, opposes that of 
the battery so that the net e. m. f. acting to move 
electrons round the circuit is no longer that of the 
battery, but is the difference between the e. m. f . of 
the battery and that of the condenser. And so, with 
each added electron, the e. m. f. of the condenser in- 
creases until finally it is just equal to that of the bat- 
tery and there is no net e. m. f . to act. 

What would happen if we should then disconnect 
the battery? The condenser would be left with its 
extra electrons in the negative plate and with its 
^^y^^^^^^y^ positive plate lacking the same num- 
ber of electrons. That is, the conden- 
ser would be left charged and its e. 
m. f. would be of the same number 
of volts as the battery. 
r/qc.^ ;^Q^ suppose we connect a short 

wire between the plates of the condenser as in Fig. 
26. The electrons rush home from the negative plate 
to the positive plate. As fast as electrons get home 



-^ 



c 




80 LETTERS OF A RADIO-ENGINEER 

the e. m. f. decreases. When they are all back the e. 
m. f. has been reduced to zero. Sometimes we say 
that *Hhe condenser discharges.'' The *' discharge 
current" starts with a rush the moment the conduct- 
ing path is offered between the two plates. The e. 
m. f. of the condenser falls, the discharge current 
grows smaller, and in a very short time the conden- 
ser is completely discharged. 

That's what happens when there is 
a short conducting path for the dis- 
charge current. If that were all that 
could happen I doubt if there would 
be any radio communication to-day. 
But if we connect a coil of wire be- 
tween two plates of a charged condenser, as in Fig. 
27, then something of great interest happens. To 
understand you must know something more about 
electron streams. 

Suppose we should wind a few turns of wire on a 
cylindrical core, say on a stiff cardboard tube. "We 
shall use insulated wire. Now start from one end of 
the coil, say a, and follow along the coiled wire for a 
few turns and then scratch off the insulation and 
solder onto the coil two wires, h, and c, as shown in 
Fig. 28. The further end of the coil we shall call 
d. Now let's arrange a battery and switch so that 
we can send a current through the part of the coil 
between a and h. Arrange also a current-measuring 
instrument so as to show if any current is flowing 
in the part of the coil between c and d. For this 
purpose we shall use a kind of current-measuring 




CONDENSERS AND COILS 81 

instrument which I have not yet explained. It is 
different from the hot-wire type described in Letter 
7 for it will show in which direction electrons are 
streaming through it. 

The diagram of Fig. 28 indicates the apparatus 
of our experiment. When we close tlie switch, S, the 
battery starts a stream of elec- 
trons from a towards h. Just at 
that instant the needle, or pointer, 
of the current instrument moves. 
The needle moves, and thus shows 
a current in the coil cd] but it 
comes right back again, showing 
/7c7 £*<5 that the current is only momen- 
tary. Let's say this again in different words. The 
battery keeps steadily forcing electrons through 
the circuit ab but the instrument in the circuit cd 
shows no current in that circuit except just at the 
instant when current starts to flow in the neighbor- 
ing circuit ah. 

One thing this current-measuring instrument tells 
us is the direction of the electron stream through 
itself. It shows that the momentary stream of elec- 
trons goes through the coil from d to c, that is in 
the opposite direction to the stream in the part ah. 
Now prepare to do a little close thinking. Eead 
over carefully all I have told you about this experi- 
ment. You see that the moment the battery starts a 
stream of electrons from a towards h, something 
causes a momentary, that is a temporary, movement 
of electrons from d to c. We say that starting a 




82 LETTERS OF A RADIO-ENGINEER 

stream of electrons from a to h sets up or *' induces'^ 
a stream of electrons from d to c. 

What will happen then if we connect the battery 
between a and d as in Fig. 29? Electrons will start 
streaming away from a tow^ards h, that is towards d. 
But that means there will be a 
momentary stream from d to- 
wards c, that is towards a. Our 
stream from the battery causes 
this oppositely directed stream. 
In the usual words we say it ' ' in- 
/^/Cp 23 duces" in the coil an opposing 
stream of electrons. This opposing stream doesn't 
last long, as we saw, but while it does last it hinders 
the stream which the battery is trying to establish. 
The stream of electrons which the battery causes 
will at first meet an opposition so it takes a little 
time before the battery can get the full-sized stream 
of electrons flowing steadily. In other words a cur- 
rent in a coil builds up slowly, because while it is 
building up it induces an effect which opposes some- 
what its own building up. 

Did you ever see a small boy start off somewhere, 
perhaps where he shouldn't be going, and find his 
conscience starting to trouble him at once. For a 
time lie goes a little slowly but in a moment or two 
his conscience stops opposing him and he goes on 
steadily at his full pace. When he started he stirred 
up his conscience and that opposed him. Nobody 
else was hindering his going. It was all brought 
about by his own actions. The opposition which he 



CONDENSERS AND COILS 83 

met was ' ^ self -induced. ' ' He was hindered at first 
by a self-induced effect of his own conscience. If 
he was a stream of electrons starting otf to travel 
around the coil we would say that he was opposed by 
a self -induced e. m. f. And any path in which such 
an effect will be produced we say has ^' self -induct- 
ance.'' Usually we shorten this term and speak of 
'inductance.'* 

There is another way of looking at it. We know 
habits are hard to form and equally hard to break. 
It's hard to get electrons going around a coil and 
the self -inductance of a circuit tells us how hard it 
is. The harder it is the more self-inductance we say 
that the coil or circuit has. Of course, we need a 
unit in which to measure self -inductance. The unit 
is called the ^' henry." But that is more self-induct- 
ance than we can stand in most radio circuits, so 
we find it convenient to measure in smaller units 
called *' mil-henries" which are thousandths of a 
henry. 

You ought to know what a henry ^ is, if we are to 
use the word, but it isn't necessary just now to 
spend much time on it. The opposition which one's 
self-induced conscience offers depends upon how 
rapidly one starts. It's volts which make electrons 
move and so the conscience which opposes them will 
be measured in volts. Therefore we say that a coil 
has one henry of inductance when an electron stream 



1 The "henry" has nothing to do with a well-kno%^Ti automobile. 
It was named after Joseph Henry, a professor years ago at Princeton 
University. 




84 LETTERS OF A RADIO-ENGINEER 

which is increasing one ampere's worth each second 
stirs up in the coil a conscientious objection of one 
volt. Don't try to remember this now; you can come 
back to it later. 

There is one more effect of in- 
ductance which we must know be- 
fore we can get very far with our 
radio. Suppose an electron stream 
is flowing through a coil because a 
-g- battei^y is driving the electrons 
along. Now let the battery be re- 
fl (^ 2Q moved or disconnected. You'd 
expect the electron stream to stop at once but it 
doesn't. It keeps on for a moment because the 
electrons have got the habit. 

If you look again at Fig. 28 you will see what I 
mean. Suppose the switch is closed and a steady 
stream of electrons is flowing through the coil from 
a to lo. There will be no current in the other part 
of the coil. Now open the switch. There will be a 
motion of the needle of the current-measuring in- 
strument, showing a momentary current. The direc- 
tion of this motion, however, shows that the momen- 
tary stream of electrons goes through the coil from 
c to d. 

Do you see what this means % The moment the bat- 
tery is disconnected there is nothing driving the elec- 
trons in the part ah and they slow do^vn. Immedi- 
ately, and just for an instant, a stream of electrons 
starts off in the part cd in the same direction as if the 
battery was driving them along. 



CONDENSERS AND COILS 



85 




Now look again at Fig. 29. If the battery is sud- 
denly disconnected there is a momentary rush of 
electrons in the same direction as the battery was 
driving them. Just as the self -inductance of a coil 
opposes the starting of a stream 
of electrons, so it opposes the 
stopping of a stream which is al- 
ready going. 

So far we haven't said much 
about making an audion produce 
/^J (^ 23 alternating e. m. f.'s and thus 
making it useful for radio-telephony. Before radio 
was possible all these things that I have just told 
you, and some more too, had to be known. It took 
hundreds of good scientists years of patient study 
and experiment to find out those ideas about elec- 
tricity which have made possible radio-telephony. 

Two of these ideas are absolutely necessary for the 
student of radio-communication. First: A conden- 
ser is a gap in a circuit where there are waiting- 
rooms for the electrons. Second: Electrons form 
habits. It's hard to get them going through a coil 
of wire, harder than through a straight wire, but 
after they are going they don't like to stop. They 
like it much less if they are going through a coil in- 
stead of a straight wire. 

In my next letter I'll tell you what happens when 
we have a coil and a condenser together in a cir- 
cuit. 



LETTER 11 



A "C-W TRANSMITTER 



Deak Son: 

Let's look again at the coils of Fig. 28 which 
we studied in the last letter. I have reproduced 
them here so you won't have to 
turn back. When electrons start 
from a towards h there is a momen- 
tary stream of electrons from d 
towards c. If the electron stream 
^•=-- through ah were started in the op- 
posite direction, that is from h 
/n C7 2Q to a the induced stream in the coil 
cd would be from c towards d. 

It all reminds me of two boys with a hedge or 
fence between them as in Fig. 30. One boy is after 





the other. Suppose you were being chased; you 
know what you'd do. If your pursuer started off 

86 



A "C-W" TRANSMITTER 



87 



with a rush towards one end of the hedge you'd 
* ' beat it ' ' towards the other. But if he started slowly 
and cautiously you would start slowly too. You 
always go in the opposite direction, dodging back 
and forth along the paths which you are wearing in 
the grass on opposite sides of the hedge. If he 
starts to the right and then slows up and starts back, 






AVOW YOU fii/^er /^fiif^r/^esr /^y?of^ 



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/vt^ 3i 



you will start to your right, slow up, and start back. 
Suppose he starts at the center of the hedge. First 
he dodges to the right, and then back through the 
center as far to the left, then back again and so on. 
You follow his every change. 

I am going to make a picture of what you two do. 
Let's start with the other fellow. He dodges or al- 
ternates back and forth. Some persons would say he 
*' oscillates" back and forth in the same path. As 



88 



LETTERS OF A RADIO-ENGINEER 



he does so he induces you to move. I am on your 
side of the hedge with a moving-picttire camera. My 
camera catches both of you. Fig. 31 shows the way 
the film would look if it caught only your heads. 
The white circle represents the tow-head on my 
side of the hedge and the black circle, young Brown 






/YOW He: iS 
/ ?F"r£:f? one C^YCLe 




who lives next door. Of course, the camera only 
catches you each time the shutter opens but it is 
easy to draw a complete picture of what takes place 
as time goes on. See Fig. 32. 

Now suppose you are an electron in coil cd of Fig. 
33 and ''Brownie" is one in coil a\ Your motions 
are induced by his. What's true of you two is true 
of all the other electrons. I have separated the coils 
a little in this sketch so that you can think of a 
hedge between. I don't know how 
one electron can affect another on 
the opposite side of this hedge but 
it can. And I don 't know anything 
really about the hedge, which is 
generally called ' ' the ether. ' ' The 
hedge isn't air. The effect would 
be the same if the coils were in 
''ether" 



?> 



r.>^ 



s -=- 



F'/Q^^ 



vacuum. The 
is just a name for whatever is left in the 
space about us when we have taken out everything 



A "C-W" TRANSMITTER 



which we can see or feel — every molecule, every pro- 
ton and every election. 

Why and how electrons can affect one another 
when they are widely separated is one of the great 
mysteries of science. We don^t know any more about 
it than about why there are electrons. Let's ac- 
cept it as a fundamental fact which we can't as yet 
explain. 

And now we can see how to make an audion pro- 
duce an alternating current or as we sometimes say 

^^make an audion oscillator.'^ 
We shall set up an audion with 
its A-battery as in Fig. 34. 
Between the grid and the fila- 
ment we put a coil and a con- 
denser. Notice that they are 
in parallel, as we say. In the 
plate-filament circuit we con- 
nect the B-battery and a switch, 
^iS', and another coil. This coil 
^ in the plate circuit of the 

audion we place close to the other coil so that the 
two coils are just like the coils ah and cd of which 
I have been telling you. The moment any current 
flows in coil ah there will be a current flow in the coil 
cd. (An induced electron stream.) Of course, as 
long as the switch in the B-battery is open no cur- 
rent can flow. 

The moment the switch S is closed the B-battery 
makes the plate positive with respect to the filament 
and there is a sudden surge of electrons round the 




90 



LETTERS OF A RADIO-ENGINEER 



plate circuit and through the coil from a to h. You 
know what that does to the coil cd. It induces an 
electron stream from d towards c. Where do these 
electrons come from? Why, from the grid and the 
plate 1 of the condenser. "WHiere do they go? Most 
of them go to the waiting-room offered by plate 2 
of the condenser and some, of course, to the filament. 
What is the result? The grid becomes positive and 
the filament negative. 

This is the crucial moment in our study. Can you 

tell me what is going to 
happen to the stream of 
electrons in the plate cir- 
cuit ? Remember that just 
at the instant when we 
closed the s^\TLtch the grid 
was neither positive nor 
negative. We were at the 
point of zero volts on the 
audion characteristic of 
Fig. 35. When we close the switch the current in the 
plate circuit starts to jump from zero mil-amperes to 
the number of mil-amperes which represents the 
point where Zero Volt St. crosses Audion Charac- 
teristic. But this jump in plate current makes the 
grid positive as we have just seen. So the grid will 
help the plate call electrons and that will make the 
current in the plate circuit still larger, that is, result 
in a larger stream of electrons from a to h. 

This increase in current will be matched by an 
increased effect in the coil cd, for you remember 





























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Pl. V. — Variometer (top) a:nd Variable Condenser (bottom) 
OF the General Radio Company. Voltmeter and Ammeter of 
THE Weston Instrument Company. 



A "C-W" TRANSMITTER 91 

how you and ^ ^Brownie'' behaved. And that will 
pull more electrons away from plate 1 of the con- 
denser and send them to the waiting-room of 2. All 
this makes the grid more positive and so makes it 
call all the more effectively to help the plate move 
electrons. 

We ''started something^' that time. It's going on 
all by itself. The grid is getting more positive, the 
plate current is getting bigger, and so the grid is 
getting more positive and the plate current still big- 
ger. Is it ever going to stop? Yes. Look at the 
audion characteristic. There comes a time when 
making the grid a little more positive won't have any 
effect on the plate-circuit current. So the plate cur- 
rent stops increasing. 

There is nothing now to keep pulling electrons 
away from plate 1 and crowding them into waiting- 
room 2. Why shouldn't the electrons in this waiting- 
room go home to that of plate 1! There is now no 
reason and so they start off with a rush. 

Of course, some of them came from the grid and 
as fast as electrons get back to the grid it becomes 
less and less positive. As the grid becomes less and 
less positive it becomes less and less helpful to the 
plate. 

If the grid doesn't help, the plate alone can't keep 
up this stream of electrons. All the plate can do 
by itself is to maintain the current represented by 
the intersection of zero volts and the audion charac- 
teristic. The result is that the current in the plate 
circuit, that is, of course, the current in coil ah, be- 



92 



LETTERS OF A RADIO-ENGINEER 



comes gradually less. About the time all the elec- 
trons, which had left the grid and plate 1 of the 
condenser, have got home the plate current is back to 
the value corresponding to Ec = 0, 

The plate current first increases and then de- 
creases, but it doesn't stop decreasing when it gets 
back to zero-grid value. And the reason is all due to 
the habit forming tendencies of electrons in coils. 
To see how this comes about, let's tell the whole story 
over again. In other words let's make a review 
and so get a sort of flying start. 

When we close the battery 
switch, S in Fig. 34, we allow 
a current to flow in the plate 
circuit. This current induces 
a current in the coil cd and 
charges the condenser which 
is across it, making plate 1 pos- 
itive and plate 2 negative. 
A positive grid helps the plate 
so that the current in the 
plate circuit builds up to the 
greatest possible value as shown by the audion char- 
acteristic. That's the end of the increase in current. 
Now the condenser discharges, sending electrons 
through the coil cd and making the grid less positive 
until finally it is at zero potential, that is neither 
positive nor negative. 

While the condenser is discharging the electrons 
in the coil cd get a habit of flowing from c toward d, 
that is from plate 2 to plate 1. If it wasn't for this 




/=7c?^^ 



A "C-W" TRANSMITTER 93 

habit the electron stream in cd would stop as soon as 
the grid had reduced to zero voltage. Because of the 
habit, however, a lot of electrons that ought to stay 
on plate 2 get hurried along and land on plate 1. 
It is a little like the old game of ''crack the whip." 
Some electrons get the habit and can^t stop quickly 
enough so they go tumbling into waiting-room 1 and 
make it negative. 

That means that the condenser not only discharges 
but starts to get charged in the other direction with 
plate 1 negative and plate 2 positive. The grid feels 
the effect of all this, because it gets extra electrons 
if plate 1 gets them. In fact the voltage effective 
between grid and filament is always the voltage be- 
tween the plates of the condenser. 

The audion characteristic tells us what is the re- 
sult. As the grid becomes negative it opposes the 
plate, shooing electrons back towards the filament 
and reducing the plate current still further. But 
you have already seen in my previous letter what 
happens when we reduce the current in coil db. 
There is then induced in coil cd an electron stream 
from c to d. This induced current is in just the 
right direction to send more electrons into waiting- 
room 1 and so to make the grid still more negative. 
And the more negative the grid gets the smaller be- 
comes the plate current until finally the plate cur- 
rent is reduced to zero. Look at the audion charac- 
teristic again and see that making the grid suffi- 
ciently negative entirely stops the plate current. 

When the plate current stops, the condenser in 



94? 



LETTERS OF A RADIO-ENGINEER 



the grid circuit is charged, with plate 1 negative and 
2 positive. It was the plate current which was the 
main cause of this change for it induced the charging 
current in coil cd. So, when the plate current be- 
comes zero there is nothing to prevent the con- 
denser from discharging. 

Its discharge makes the grid less and less negative 
until it is zero volts and there we are — back practi- 
cally where we started. The plate current is increas- 
ing and the grid is getting 
positive, and we're off on 
another *' cycle" as we say. 
During a cycle the plate cur- 
rent increases to a maximum, 
decreases to zero, and then 
increases again to its initial 
value. 

This letter has a longer 
continuous train of thought 
than I usually ask you to 
follow. But before I stop I want to give you some 
idea of what good this is in radio. 

What about the current which flows in coil cd? 
It's an alternating current, isn't it? First the elec- 
trons stream from d towards c, and then back again 
from c towards d. 

Suppose we set up another coil like CD in Fig. 36. 
It would have an alternating current induced in it. 
If this coil was connected to an antenna there would 
be radio waves sent out. The switch 8 could be used 
for a key and kept closed longer or shorter intervals 







A "C-W" TRANSMITTER 



95 



depending upon whether dashes or dots were being 
set. I'll tell you more about this later, but in this 
diagram are the makings of a **C-W Transmitter/' 
that is a ** continuous wave transmitter'' for radio- 
telegraphy. 

It would be worth while to go over this letter again 
using a pencil and tracing in the various circuits 
the electron streams which I have described. 



LETTER 12 
INDUCTANCE AND CAPACITY 

Deak Sie : 

In the last letter I didn't stop to draw you a pic- 
ture of the action of the audion oscillator which I 
described. I am going to do it now and you are to 
imagine me as using two pencils and drawing simul- 
taneously two curves. One curve shows what hap- 
pens to the current in the plate circuit. The other 
shows how the voltage of the grid changes. Both 
curves start from the instant when the switch is 
closed; and the two taken together show just what 
happens in the tube from instant to instant. 

Fig. 37 shows the two curves. You will notice how 
I have drawn them beside and below the audion char- 
acteristic. The grid voltage and the plate current 
are related, as I have told you, and the audion char- 
acteristic is just a convenient way of showing the 
relationship. If we know the current in the plate 
circuit we can find the voltage of the grid and vice 
versa. 

As time goes on, the plate current grows to its 
maximum and decreases to zero and then goes on 
climbing up and down between these two extremes. 
The grid voltage meanwhile is varying alternately, 
having its maximum positive value when the plate 
current is a maximum and its maximum negative 

96 



INDUCTANCE AND CAPACITY 



97 



value when the plate current is zero. Look at the 
two curves and see this for yourself. 

Now I want to tell you something about how fast 
these oscillations occur. We start by learning two 
words. One is ^^ cycle'' with which you are already 
partly familiar and the other is ^ ^frequency.'' Take 



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voL.T/'icse: 



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cycle first. Starting from zero the current increases 
to a maximum, decreases to zero, and is ready 
again for the same series of changes. We say 
the current has passed through ^^a cycle of values.'' 
It doesn't make any difference where we start from. 
If we follow the current through all its different 
values until we are back at the same value as we 
started with and ready to start all over, then we 
have followed through a cycle of values. 



98 LETTERS OF A RADIO-ENGINEER 

Once you get the idea of a cycle, and the mark- 
ings on the curves in Fig. 31 will help you to under- 
stand, then the other idea is easy. By "frequency'^ 
we mean the number of cycles each second. The 
electric current which we use in lighting our house 
goes through sixty cycles a second. That means 
the current reverses its direction 120 times a 
second. 

In radio we use alternating currents which have 
very high frequencies. In ship sets the frequency 
is either 500,000 or 1,000,000 cycles per second. 
Amateur transmitting sets usually have oscillators 
which run at well over a million cycles per second. 
The longer range stations use lower frequencies. 

You'll find, however, that the newspaper an- 
nouncements of the various broadcast stations do not 
tell the frequency but instead tell the ^ ^ wave length. ' ' 
1 am not going to stop now to explain what that 
means but I am going to give you a simple rule. 
Divide 300,000,000 by the ^^wave length" and you'll 
have the frequency. For example, ships are sup- 
posed to use wave lengths of 300 meters or 600 
meters. Dividing three hundred million by three 
hundred gives one million and that is one of the fre- 
quencies which I told you were used by ship sets. 
Dividing by six hundred gives 500,000 or just half 
the frequency. You can remember that sets trans- 
mitting with long waves have low frequencies, but 
sets with short waves have high frequencies. The 
frequency and the wave length don't change in the 
same way. They change in opposite ways or in- 




INDUCTANCE AND CAPACITY 99 

versely, as we say. The higher the frequency the 
shorter the wave length. 

I'll tell you about wave lengths later. First let's 
see how to control the frequency of an audion oscil- 
lator like that of Fig. 38. 

It takes time to get a full-sized stream going 
through a coil because of the inductance of the coil. 
That you have learned. And also it takes time for 
such a current to stop com- 
pletely. Therefore, if we make 
the inductance of the coil small, 
keeping the condenser the same, 
we shall make the time required 
for the current to start and stop 
smaller. That will mean a higher F'/qOS 
frequency for there will be more oscillations each 
second. One rule, then, for increasing the frequency 
of an audion oscillator is to decrease the inductance. 

Later in this letter I shall tell you how to increase 
or decrease the inductance of a coil. Before I do so, 
however, I want to call your attention to the other 
way in which we can change the frequency of an 
audion oscillator. 

Let's see how the frequency will depend upon the 
capacity of the condenser. If a condenser has a 
large capacity it means that it can accommodate 
in its waiting-room a large number of electrons be- 
fore the e. m. f. of the condenser becomes large 
enough to stop the stream of electrons which is 
charging the condenser. If the condenser in the 
grid circuit of Fig. 38 is of large capacity it means 



100 LETTERS OF A RADIO-ENGINEER 

that it must receive in its upper waiting-room 
a large number of electrons before tbe grid will be 
liegative enough to make the plate current zero. 
Therefore, the charging current will have to flow 
a long time to store up the necessary number of 
electrons. 

You will get the same idea, of course, if you think 
about the electrons in the lower room. The current 
in the plate circuit will not stop increasing until 
the voltage of the grid has become positive enough 
to make the plate current a maximum. It can't do 
that until enough electrons have left the upper room 
and been stored away in the lower. Therefore the 
charging current will have to flow for a long time if 
the capacity is large. We have, therefore, the other 
rule for increasing the frequency of an audion os- 
cillator, that is, decrease the capacity. 

These rules can be stated the other way around. 
To decrease the frequency we can either increase 
the capacity or increase the inductance or do both, i 

But what would happen if we should decrease the 
capacity and increase the inductance! Decreasing 
the capacity would make the frequency higher, but 
increasing the inductance would make it lower. 
What would be the net effect! That would depend 
upon how much we decreased the capacity and how 
much we increased the inductance. It would be pos- 
sible to decrease the capacity and then if we in- 
creased the inductance just the right amount to 
have no change in the frequency. No matter how 
large or how small we make the capacity we can 



INDUCTANCE AND CAPACITY 101 

always make the inductance such that there isn't 
any change in frequency. I'll give you a rule for 
this, after I have told you some more things about 
capacities and inductances. 

First as to inductances. A short straight wire 
has a very small inductance, indeed. The longer the 
wire the larger will be the inductance but unless 
the length is hundreds of feet there isn't much in- 
ductance anyway. A coiled wire is very different. 

A coil of wire will have more inductance the more 
turns there are to it. That isn't the whole story 
but it's enough for the moment. Let's see why. The 
reason why a stream of electrons has an opposing 
conscience when they are started off in a coil of 
wire is because each electron affects every other 
electron which can move in a parallel path. Look 
again at the coils of Figs. 28 and 29 which we dis- 
cussed in the tenth letter. Those sketches plainly 
bring out the fact that the electrons in part cd 
travel in paths which are parallel 
to those of the electrons in part ah. 

If we should turn these coils as 
in Fig. 39 so that all the paths in 
cd are at right angles to those in ah 





there wouldn't be any effect in c<i ^ ^ 

when a current in ah started or /v^ s3^ 
stopped. Look at the circuit of the oscillating 
audion in Fig. 38. If we should turn these coils at 
right angles to each other we would stop the oscil- 
lation. Electrons only influence other electrons 
which are in parallel paths. 



102 LETTERS OF A RADIO-ENGINEER 

When we want a large inductance we wind the coil 
so that there are many parallel paths. Then when 
the battery starts to drive an electron along, this 
electron affects all its fellows who are in parallel 
paths and tries to start them off in the opposite 
direction to that in which it is being driven. The bat- 
tery, of course, starts to drive all the electrons, not 
only those nearest its negative terminal but those 
all along the wire. And every one of these electrons 
makes up for the fact that the battery is driving it 
along by urging all its fellows in the opposite direc- 
tion. 

It is not an exceptional state of affairs. Suppose 
a lot of boys are being driven out of a yard where 
they had no right to be playing. Suppose also that 
a boy can resist and lag back twice as much if some 
other boy urges him to do so. Make it easy and 
imagine three boys. The first boy lags back not only 
on his own account but because of the urging of 
the other boys. That makes him three times as 
hard to start as if the other boys didn't influence 
him. The same is true of the second boy and also of 
the third. The result is the unfortunate property 
owner has nine times as hard a job getting that 
gang started as if only one boy were to be dealt 
with. If there were two boys it would be four 
times as hard as for one boy. If there were four in 
the group it would be sixteen times, and if five it 
would be twenty-five times. The difficulty increases 
much more rapidly than the number of boys. 

Now all we have to do to get the right idea of in- 



INDUCTANCE AND CAPACITY 



103 



ductance is to think of each boy as standing for the 
electrons in one turn of the coil. If there are five 
turns there will be twenty-five times as much induct- 
ance, as for a single turn; and so on. You see that 
we can change the inductance of a coil very easily by 
changing the number of turns. 

I'll tell you two things more about inductance be- 
cause they will come in handy. The first is that the 
inductance will be larger if the turns are large 
circles. You can see that for yourself because if the 
circles were very small we would have practically a 
straight wire. 

The other fact is this. If that property owner 
had been an electrical engineer and the boys had 
been electrons he would have fixed it so that while 
half of them said, ^^Aw, don't go; he can't put you 
off"; the other half would have said *^Come on, let's 
get out. " If he did that he would have a coil without 
any inductance, that is, he would have only the nat- 
ural inertia of the electrons to deal with. We would 
say that he had made a coil with *^pure resistance" 
or else that he had made a ^* non-induc- 
tive resistance." 

How would he do it? Easy enough 
after one learns how, but quite in- 
genious. Take the wire and fold it at 
the middle. Start with the middle and /=''/q ^o 
wind the coil with the doubled wire. Fig. 40 shows 
how the coil would look and you can see that part of 
the way the electrons are going around the coil in 
one direction and the rest of the way in the opposite 




104 LETTERS OF A RADIO-ENGINEER 

direction. It is just as if the boys were paired off, 
a ^'goody-goody'' and a ''tough nuf together. 
They both shout at once opposite advice and neither 
has any effect. 

I have told you all except one of the ways in which 
we can affect the inductance of a circuit. You know 
now all the methods which are important in radio. 
So let's consider how to make large or small capaci- 
ties. 

First I want to tell you how we measure the ca- 
pacity of a condenser. "We use units called ''micro- 
farads.'' You remember that an ampere means an 
electron stream at the rate of about six billion billion 
electrons a second. A millionth of an ampere would, 
therefore, be a stream at the rate of about six million 
million electrons a second — quite a sizable little 
stream for any one who wanted to count them as they 
went by. If a current of one millionth of an ampere 
should flow for just one second six million million 
electrons would pass along by every point in the path 
or circuit. 

That is what would happen if there weren't any 
waiting-rooms in the circuit. If there was a con- 
denser then that number of electrons would leave 
one waiting-room and would enter the other. Well, 
suppose that just as the last electron of this enor- 
mous number ' entered its waiting-room we should 
know that the voltage of the condenser was just one 
volt. Then we would say that the condenser had a ca- 
pacity of one microfarad. If it takes half that num- 

1 More accurately the number is 6,286,000,000,000. 



INDUCTANCE AND CAPACITY 105 

ber to make the condenser oppose further changes 
in the contents of its waiting-rooms, with one volt's 
worth of opposition, that is, one volt of e. m. f., then 
the condenser has only half a microfarad of capacity. 
The number of microfarads of capacity (abbreviated 
mf.) is a measure of how many electrons we can 
get away from one plate and into the other before 
the voltage rises to one volt. 

What must we do then to make a condenser with 
large capacity? Either of two things; either make 
the waiting-rooms large or put them close together. 

If we make the plates of a condenser larger, keep- 
ing the separation between them the same, it means 
more space in the waiting-rooms and hence less 
crowding. You know that the more crowded the 
electrons become the more they push back against 
any other electron which some battery is trying to 
force into their waiting-room, that is the higher the 
e. m. f. of the condenser. 

The other way to get a larger capacity is to bring 
the plates closer together, that is to shorten the gap. 
Look at it this way: The closer the plates are to- 
gether the nearer home the electrons are. Their 
home is only just across a little gap ; they can almost 
see the electronic games going on around the nuclei 
they left. They forget the long round-about journey 
they took to get to this new waiting-room and they 
crowd over to one side of this room to get just as 
close as they can to their old homes. That's why it's 
always easier, and takes less voltage, to get the same 
number of electrons moved from one plate to the 



106 LETTERS OF A RADIO-ENGINEER 

other of a condenser whicli has only a small space 
between plates. It takes less voltage and that means 
that the condenser has a smaller e. m. f. for the 
same number of electrons. It also means that before 
the e. m. f . rises to one volt we can get more electrons 
moved around if the plates are close together. And 
that means larger capacity. 

There is one thing to remember in all this : It 
doesn't make any difference how thick the plates 
are. It all depends upon how much surface they have 
and how close together they are. Most of the elec- 
trons in the plate which is being made negative are 
way over on the side toward their old homes, that 
is, toward the plate which is being made positive. 
And most of the homes, that is, atoms which have 
lost electrons, are on the side of the positive plate 
which is next to the gap. That 's why I said the elec- 
trons could almost see their old homes. 

All this leads to two very simple rules for build- 
ing condensers. If you have a condenser with too 
small a capacity and want one, say, twice as large, 
you can either use twice as large plates or bring 
the plates you already have twice as close together; 
that is, make the gap half as large. Generally, of 

1 \ course, the gap is pretty well fixed. 



I For example, if we make a con- 
Ljiiij denser by using two pieces of metal 

I r f^/Q^I and separating them by a sheet of 
mica we don't want the job of splitting the mica. 
So we increase the size of the plates. We can do 
that either by using larger plates or other plates 





Pl. VI. — Low-power Transmitting Tube, U V 202 (Courtesy of 
Radio Corporation of Aimerica). 



INDUCTANCE AND CAPACITY 



107 



.± t t 1-. 



±^ 



HiHTt^ 



and connecting it as in Fig. 41 so that the total 
waiting-room space for electrons is increased. 
If you have got these ideas you can understand 

how we use both sides of the _, , 

same plate in some types of 
condensers. Look at Fig. 42. 
There are two plates connected 
together and a third between 
them. Suppose electrons are pulled from the out- 
side plates and crowded into the middle plate. 
Some of them go on one side and some on the 
other, as I have shown. The 
negative signs indicate electrons 
and the plus signs their old 
homes. If we use more plates 
as in Fig. 43 we have a larger 
capacity. 

"What if we have two plates 
which are not directly opposite 
one another, like those of Fig. 44! What does the 
capacity depend upon? Imagine yourself an elec- 
tron on the negative plate. Look otf toward the 
positive plate and see how ^ 

big it seems to you. The 
bigger it looks the more ca- 
pacity the condenser has. 
When the plates are right 
opposite one another the posi- 
tive plate looms up pretty 
large. But if they slide apart you don't see so 
much of it ; and if it is off to one side about all you 



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108 LETTERS OF A RADIO-ENGINEER 

see is the edge. If you can't see lots of atoms which 
have lost electrons and so would make good homes 
for you, there is no use of your staying around on 
that side of the plate; you might just as well be 
trying to go back home the long way which you 
originally came. 

That's why in a variable plate condenser there is 
very little capacity when no parts of the plates are 
opposite each other, and there is the greatest ca- 
pacity when they are exactly opposite one another. 

While we are at it we might just as well clean up 
this whole business of variable capacities and induct- 
ances by considering two ways 
in which to make a variable 
inductance. Fig. 45 shows the 
F'/C^ ^3 simplest way but it has some 
disadvantages which I won't try now to explain. 
We make a long coil and then take off taps. We 
can make connections between one end of the coil 
and any of the taps. The more turns there are 
included in the part of the coil which we are using 
the greater is the inductance. If we want to do a 
real job we can bring each of these taps to a little 
stud and arrange a sliding or rotating contact with 
them. Then we have an inductance the value of 
which we can vary ^ ' step-by-step " in a convenient 
manner. 

Another way to make a variable inductance is to 
make what is called a "variometer." I dislike the 
name because it doesn't "meter" anything. If 
properly calibrated it would of course "meter" in- 



INDUCTANCE AND CAPACITY 109 

ductance, but then it should be called an ^'inducto- 
meter. ' ' 

Do you remember the gang of boys that fellow 
had to drive off his property? What if there had 
been two different gangs playing there ? How much 
trouble he has depends upon whether there is any- 
thing in common between the gangs. Suppose they 
are playing in different parts of his property and 
so act just as if the other crowd wasn^t also trespass- 
ing. He could just add the trouble of starting one 
gang to the trouble of starting the other. 

It would be very different if the gangs have any- 
thing in common. Then one would encourage the 
other much as the various boys of the same gang 
encourage each other. He would have a lot more 
trouble. And this extra trouble would be because 
of the relations between gangs, that is, because of 
their *' mutual inductance." 

On the other hand suppose the gangs came from 
different parts of the town and disliked each other. 
He wouldn't have nearly the trouble. Each gang 
would be yelling at the other as they went along: 
*^ You'd better beat it. He knows all right, all right, 
who broke that bush down by the gate. Just wait 
till he catches you." They'd get out a little easier, 
each in the hope the other crowd would catch it from 
the owner. There's a case where their mutual rela- 
tions, their mutual inductance, makes the job easier. 

That's true of coils with inductance. Suppose you 
wind two inductance coils and connect them in series. 
If they are at right angles to each other as in Fig. 



110 LETTERS OF A RADIO-ENGINEER 




46a they have no effect on each other. There is no 
mutual inductance. But if they are 
parallel and wound the same way 
like the coils of Fig. 46b they will 

^ ^ act like a single coil of greater in- 

(/(/(/{/ ductance. If the coils are parallel 

^^^-^^-'^^^ but wound in opposite directions as 

A/^ ^^ '^ in Fig. 46c they will have less in- 
ductance because of their mutual inductance. You 
can check these statements for yourself if you'll 
refer back to Letter 10 and see what 
happens in the same way as I told 
you in discussing Fig. 28. 

If the coils are neither parallel nor 
at right angles there will be some mu- 
tual inductance but not as much as if 
they were parallel. By turning the 
coils we can get all the variations in 
mutual relations from the case of Fig. 46b to that 
of Fig. 46c. That's what we arrange to do in a 
variable inductance of the vario- 
meter type. 

There is another way of varying 
the mutual inductance. We can 
make one coil slide inside another. 
If it is way inside, the total in- 
ductance which the two coils offer 
either larger than the sum of 




/=7^ ^^ a 




IS eitner larger tnan tne sum 
what they can offer separately or 



less, depending upon whether the windings are in 



INDUCTANCE AND CAPACITY 



111 



the same direction or opposite. As we pull the coil 
out the mutual effect becomes less and finally when 
it is well outside the mutual inductance is very 
small. 

Now we have several methods of varying capacity 
and inductance and therefore we are ready to vary 
the frequency of our audion oscillator; that is, 
^'tune'' it, as we say. In my next letter I shall 
show you why we tune. 

Now for the rule which I promised. The fre- 
quency to which a circuit is tuned depends upon the 
product of the number of mil-henries in the coil and 
the number of micro-farads in the condenser. 
Change the coil and the condenser as much as you 
want but keep this product the same and the fre- 
quency will be the same. 



LETTER 13 



TUNING 



/f 




Deak Eadio Enthusiast : 

I want to tell you about receiving sets and their 
tuning. In the last letter I told you what determines 
the frequency of oscillation of an audion oscillator. 
It was the condenser and inductance 
which you studied in connection with 
Fig. 36. That's what determines the 
frequency and also what makes the os- 
cillations. All the tube does is to keep 
them going. Let's see why this is so. 
Start first, as in Fig. 47a, with a very simple cir- 
cuit of a battery and a non-inductive resistance, that 
is, a wire wound like that of Fig. 40 in the previous 
letter, so that it has no inductance. The battery 
must do work forcing electrons through that wire. 
It has the ability, or the energy as we say. 

Now connect a condenser to the battery as in Fig. 
47b. The connecting wires are very 
short; and so practically all the work 
which the battery does is in storing 
electrons in the negative plate of the 
condenser and robbing the positive 
plate. The battery displaces a certain rtQ^7& 
number of electrons in the waiting-rooms of the 
condenser. How many, depends upon how hard it 

112 



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C 



TUNING 



113 



can push and pull, that is on its e. m. f ., and upon 
how much capacity the condenser has. 

Eemove the battery and connect the charged con- 
denser to the resistance as in Fig. 47c. The elec- 
trons rush home. They bump and 
jostle their way along, heating the 
wire as they go. They have a certain 
amount of energy or ability to do work ^--'^VWWAAH 
because they are away from home and /^/<^ ^7 c 
they use it all up, bouncing along on their way. 
When once they are home they have used up all the 
surplus energy which the battery gave them. 

Try it again, but this time, as in Fig. 47d, connect 
the charged condenser to a coil which has inductance. 
The electrons don't get started as fast because 
of the inductance. But they keep going because the 
electrons in the wire form the habit. The result is 
that about the time enough electrons have got into 
plate 2 (which was positive), to satisfy all its lonely 
protons, the electrons in the wire are streaming 
along at a great rate. A lot of them keep going until 
they land on this plate and so make it negative. 

That's the same sort of thing that happens in 
the case of the inductance and con- 
denser in the oscillating audion circuit 
except for one important fact. There 
xiuuOuU is nothing to keep electrons going to 
/=7^ ^7o the 2 plate except this habit. And 
there are plenty of stay-at-home electrons to stop 
them as they rush along. They bump and jostle, 
but some of them are stopped or else diverted so 



114 LETTERS OF A RADIO-ENGINEER 

that they go bumping around without getting any 
nearer plate 2. Of course, they spend all their 
energy this way, getting every one all stirred up 
and heating the wire. 

Some of the energy which the electrons had when 
they were on plate 1 is spent, therefore, and there 
aren't as many electrons getting to plate 2. When 
they turn around and start back, as you know they 
do, the same thing happens. The result is that each 
successive surge of electrons is smaller than the 
preceding. Their energy is being wasted in heating 
the wire. The stream of elec- 

rlllllllllllh ^^^^^ ^^^^ smaller and smaller, 

^) and the voltage of the con- 

nnon (\f)f\f)(\ \ ^^^^^^ S^^^ smaller and smaller, 

JuMJIJ^^ mtil by-and-by there isn't any 

ei c £- CK. stream and the condenser is 

f/q ^CD 2g£|. -Qncharged. When that 

happens, we say the oscillations have ^'damped 

out.'' 

That's one way of starting oscillations which 
damp out — to start with a charged condenser and 
connect an inductance across it. There is another 
way which leads us to some important ideas. Look 
at Fig. 48. There is an inductance and a condenser. 
Near the coil is another coil which has a battery and 
a key in circuit with it. The coils are our old friends 
of Fig. 33 in Letter 10. Suppose we close the switch 
S. It starts a current through the coil ah which goes 
on steadily as soon as it really gets going. While 
it is starting, however, it induces an electron stream 



TUNING 



115 



in coil cd. There is only a momentary or transient 
current bnt it serves to charge the condenser and 
then events happen just as they did in the case where 
we charged the condenser with a battery. 

Now take away this coil ah with its battery and 
substitute the oscillator of Fig. 36. What's going 




to happen? We have two circuits in which oscilla- 
tions can occur. See Fig. 49. One circuit is asso- 
ciated with an audion and some batteries which 
keep supplying it with energy so that its oscilla- 
tions are continuous. The other circuit is near 
enough to the first to be influenced by what happens 
in that circuit. We say it is * ^coupled'' to it, be- 
cause whatever happens in the first circuit induces 
an effect in the second circuit. 

Suppose first that in each circuit the inductance 
and capacity have such values as to produce oscilla- 
tions of the same frequency. Then the moment we 
start the oscillator we have the same effect in both 
circuits. Let me draw the picture a little differently 
(Fig. 50) so that you can see this more easily. I 
have merely made the coil db in two parts, one of 
which can affect cd in the oscillator and the other 
the coil L of the second circuit. 



116 



LETTERS OF A RADIO-ENGINEER 



But suppose that the two circuits do not have 
the same natural frequencies, that is the condenser 




T— -liHttH 



r/g30 

and inductance in one circuit are so large that it just 
naturally takes more time for an oscillation in that 
circuit than in the other. It is like learning to dance. 
You know about how well you and your partner 
would get along if you had one frequency of oscil- 
lation and she had another. That's what happens 
in a case like this. 

If circuit L-C takes longer for each oscillation 
than does circuit ah its electron stream is always 
working at cross purposes with the electron stream 
in ab which is trying to lead it. Its electrons start 
off from one condenser plate to the other and be- 
fore they have much more than got started the 
stream in ah tries to call them back to go in the 
other direction. It is practically impossible under 
these conditions to get a stream of any size going 
in circuit L-C. It is equally hard if L-C has smaller 
capacity and inductance than ab so that it naturally 
oscillates faster. 

I'll tell you exactly what it is like. Suppose you 
and your partner are trying to dance without any 
piano or other source of music. She has one tune 
running through her head and she dances to that, 



TUNING 



117 



except as you drag her around the floor. You are 
trying to follow another tune. As a couple you have 
a difficult time going anywhere under these condi- 
tions. But it would be all right if you both had the 
same tune. 

If we want the electron stream in coil ah to have 
a large guiding effect on the stream in coil L-C we 
must see that both 
circuits have the 
same tune, that is 
the same natural 
frequency of oscilla- 
tion. 

This can be shown 
very easily by a sim- 
ple experiment. Sup- 
pose we set up our 
circuit L-C with an 
ammeter in it, so as 
to be able to tell 
how large an elec- 
tron stream is oscil- 
lating in that circuit. Let us also make the con- 
denser a variable one so that we can change the 
natural frequency or tune of the circuit. Now 
let's see what happens to the current as we vary 
this condenser, changing the capacity and thus 
changing the tune of the circuit. If we use a 
variable plate condenser it will have a scale on top 
graduated in degrees and we can note the reading 
of the ammeter for each position of the movable 









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118 LETTERS OF A RADIO-ENGINEER 



plates. If we do, we find one position of these 
plates, that is one setting, corresponding to one 
value of capacity in the condenser, where the current 
in the circuit is a maximum. This is the setting 
of the condenser for which the circuit has the same 
tune or natural frequency as the circuit cd. Some- 
times we say that the circuits are now in resonance. 
"We also refer to the curve of values of current and 
condenser positions as a ''tuning curve.'' Such a 
curve is shown in Fig. 51. 




Mill 
|ili|i|i|i|-J 



/^-/Qsa 



That's all there is to tuning — adjusting the ca- 
pacity and inductance of a circuit until it has the 
same natural frequency as some other circuit with 
which we want it to work. We can either adjust 
the capacity as we just did, or we can adjust the in 
ductance. In that case we use a variable inductance 
as in Fig. 52. 

If we want to be able to tune to any of a large 
range of frequencies we usually have to take out or 
put into the circuit a whole lot of mil-henries at a 
time. When we do we get these mil-henries of in- 
ductance from a coil which we call a ''loading coil." 
That's why your friends add a loading coil when they 



'-\ 



TUNING 



119 



want to tune for the long wave-length stations, that 
is, those with a low frequency. 

When our circuit L-C of Fig. 49 is tuned to the 
frequency of the oscillator we get in it a maximum 
current. There is a maximum stream of electrons, 
and hence a maximum number of them crowded first 
into one and then into the other plate of the con- 
denser. And so the condenser is charged to a max- 
imum voltage, first in one direction and then in the 
other. 







Now connect the circuit L-C to the grid of an 
audion. If the circuit is tuned we'll have the max- 
imum possible voltage applied between grid and fila- 
ment. In the plate circuit we'll get an increase and 
then a decrease of current. You know that will hap- 
pen for I prepared you for this moment by the last 
page of my ninth letter. I'll tell you more about that 
current in the plate circuit in a later letter. I am 
connecting a telephone receiver in the plate circuit, 
and also a condenser, the latter for a reason to be 
explained later. The combination appears then as in 
Fig. 53. That figure shows a C-W transmitter and 
an audion detector. This is the sort of a detector 



120 



LETTERS OF A RADIO-ENGINEER 



we would use for radio-telephony, but the trans- 
mitter is the sort we would use for radio-telegraphy. 
We shall make some changes in them later. 

Whenever we start the oscillating current in the 
transmitter we get an effect in the detector circuit, 
of which I'll tell you more later. For the moment 
I am interested in showing you how the transmitter 
and the detector may be separated by miles and 




TSL£PHOrfS 
HE/JO S£T 







'Cor/OS'r/SS'/^ 



still there will be an effect in the detector circuit 
every time the key in the transmitter circuit is 
closed. 

This is how we do it. At the sending station, that 
is, wherever we locate the transmitter, we make a 
condenser using the earth, or ground, as one plate. 
We do the same thing at the receiving station where 
the detector circuit is located. To these condensers 
we connect inductances and these inductances we 
couple to our transmitter and receiver as shown in 
Fig. 54. The upper plate of the condenser in each 



TUNING 121 

case is a few horizontal wires. The lower plate is 
the moist earth of the ground and we arrange to get 
in contact with that in various ways. One of the 
simplest methods is to connect to the water pipes 
of the city water-system. 

Now we have our radio transmitting-station and a 
station for receiving its signals. You remember we 
can make dots and dashes by the key or switch 
in the oscillator circuit. When we depress the key 
we start the oscillator going. That sets up oscilla- 
tions in the circuit with the inductance and the ca- 
pacity formed by the antenna. If we want a real- 
sized stream of electrons up and down this antenna 
lead (the vertical wire), we must tune that circuit. 
That is why I have shown a variable inductance in 
the circuit of the transmitting antenna. 

What happens when these electrons surge back 
and forth between the . horizontal wires and the 
ground, I don't know. I do know, however, that if 
we tune the antenna .circuit at the receiving station 
there will be a small stream of electrons surging 
back and forth in that circuit. 

Usually scientists explain what happens by saying 
that the transmitting station sends out waves in 
the ether and that these waves are received by the 
antenna system at the distant station. Wherever you 
put up a receiving station you will get the effect. 
It will be much smaller, however, the farther the 
two stations are apart. 

I am not going to tell you anything about wave 
motion in the ether because I don't believe we know 



122 LETTERS OF A RADIO-ENGINEER 

enough about the ether to try to explain, but I shaU 
tell you what we mean by ' ' wave length. ' ' 

Somehow energy, the ability to do work, travels 
out from the sending antenna in all directions. 
Wherever you put up your receiving station you 
get more or less of this energy. Of course, energy 
is being sent out only while the key is depressed and 
the oscillator going. This energy travels just as 
fast as light, that is at the enormous speed of 186,000 
miles a second. If you use meters instead of miles 
the speed is 300,000,000 meters a second. 

Now, how far will the energy which is sent out 
from the antenna travel during the time it takes 
for one oscillation of the current in the antenna? 
Suppose the current is oscillating one million times 
a second. Then it takes one-millionth of a second 
for one oscillation. In that time the energy will 
have traveled away from the antenna one-millionth 
part of the distance it will travel in a whole second. 
That is one-millionth of 300 million meters or 300 
meters. 

The distance which energy will go in the time 
taken by one oscillation of the source of that energy 
is the wave length. In the case just given that dis- 
tance is 300 meters. The wave length, then, of 300 
meters corresponds to a frequency of one million. 
In fact if we divide 300 million meters by the fre- 
quency we get the wave length, and that's the same 
rule as I gave you in the last letter. 

In further letters I'll tell you how the audion 
works as a detector and how we connect a telephone 



TUNING 



123 



transmitter to the oscillator to make it send out 
energy with a speech significance instead of a mere 
dot and dash significance, or signal significance. We 
shall have to learn quite a little about the telephone 
itself and about the human voice. 



mn 



LETTER 14 

WHY AND HOW TO USE A DETECTOR 

Deae Son : 

In the last letter we got far enough to sketch, in 
Fig. 54, a radio transmitting station and a receiving 
station. We should never, however, use just this 
combination because the transmitting station is in- 




-L- -± 



— \ 



TSLEPHONC 
HEAD S£r 







tended to send telegraph signals and the receiving 
station is best suited to receiving telephonic trans- 
mission. But let us see what happens. 

When the key in the plate circuit of the audion at 
the sending station is depressed an alternating cur- 
rent is started. This induces an alternating current 
in the neighboring antenna circuit. If this antenna 
circuit, which is formed by a coil and a condenser, 
is tuned to the frequency of oscillations which are 

124 



WHY WE USE A DETECTOR 125 

being produced in the audion circuit then there is a 
maximum current induced in the antenna. 

As soon as this starts the antenna starts to send 
out energy in all directions, or ** radiate" energy 
as we say. How this energy, or ability to do work, 
gets across space we don't know. However it may 
be, it does get to the receiving station. It only takes 
a small fraction of a second before the antenna at the 
receiving station starts to receive energy, because 
energy travels at the rate of 186,000 miles a second. 

The energy which is received does its work in mak- 
ing the electrons in that antenna oscillate back and 
forth. If the receiving antenna is tuned to the fre- 
quency which the sending station is producing, then 
the electrons in the receiving antenna oscillate back 
and forth most widely and there is a maximum cur- 
rent in this circuit. 

The oscillations of the electrons in the receiving 
antenna induce similar oscillations in the tuned cir- 
cuit which is coupled to it. This circuit also is tuned 
to the frequency which the distant oscillator is pro- 
ducing and so in it we have the maximum oscilla- 
tion of the electrons. The condenser in that circuit 
charges and discharges alternately. 

The grid of the receiving audion always has the 
same voltage as the condenser to which it is con- 
nected and so it becomes alternately positive and 
negative. This state of affairs starts almost as soon 
as the key at the sending station is depressed and 
continues as long as it is held down. 

Now what happens inside the audion? As the 



126 LETTERS OF A RADIO-ENGINEER 

grid becomes more and more positive the current 
in the plate circuit increases. When the grid no 
longer grows more positive but rather becomes less 
and less positive the current in the plate circuit de- 
creases. As the grid becomes of zero voltage and 
then negative, that is as the grid ^^ reverses its pol- 
arity,'' the plate current continues to decrease. 
When the grid stops growing more negative and 
starts to become less so, the plate current stops de- 
creasing and starts to increase. 

All this you know, for you have followed through 
such a cycle of changes before. You know also how 
we can use the audion characteristic to tell us what 
sort of changes take place in the plate current when 
the grid voltage changes. The plate current in- 
creases and decreases alternately, becoming greater 
and less than it would be if the grid were not inter- 
fering. These variations in its intensity take place 
very rapidly, that is with whatever high frequency 
the sending station operates. What happens to the 
plate current on the average? 

The plate current, you remember, is a stream of 
electrons from the filament to the plate (on the in- 
side of the tube), and from the plate back through 
the B-battery to the filament (on the outside of the 
tube). The grid alternately assists and opposes that 
stream. When it assists, the electrons in the plate 
circuit are moved at a faster rate. When the grid 
becomes negative and opposes the plate the stream 
of electrons is at a slower rate. The stream is al- 
ways going in the same direction but it varies in its 



WHY WE USE A DETECTOR 



127 



rate depending upon the changes in grid potential. 
When the grid is positive, that is for half a cycle 
of the alternating grid-voltage, the stream is larger 
than it would be if the plate current depended only 
on the B-battery. For the other half of a cycle it 
is less. The question I am raising is this : Do more 
electrons move around the plate circuit if there is 










GR/a volt^qe: 



to^7V9^£: 



F'/q^^ 



a signal coming in than when there is no incoming 
signal? To answer this we must look at the audion 
characteristic of our particular tube and this char- 
acteristic must have been taken with the same B- 
battery as we use when we try to receive the signals. 
There are just three possible answers to this ques- 
tion. The first answer is: ^'No, there is a smaller 
number of electrons passing through the plate cir- 
cuit each second if the grid is being affected by an 
incoming signal." The second is: ^^The signal 



128 LETTERS OF A RADIO-ENGINEER 

doesn't make any difference in the total number of 
electrons which move each second from filament to 
plate." And the third answer is: "Yes, there is a 
greater total number each second." 

Any one of the three answers may be right. It 
all depends on the characteristic of the tube as we 
are operating it, and that depends not only upon 
the type and design of tube but also upon what volt- 
ages we are using in our batteries. Suppose the var- 







iations in the voltage of the grid are as represented 
in Fig. 55, and that the characteristic of the tube 
is as shown in the same figTire. Then obviously the 
first answer is correct. You can see for yourself 
that when the grid becomes positive the current in 
the plate circuit can't increase much anyway. For 
the other half of the cycle, that is, while the grid is 
negative, the current in the plate is very much de- 
creased. The decrease in one half-cycle is larger 
than the increase during the other half-cycle, so 
that on the avera2:e the current is less when the 



WHY WE USE A DETECTOR 



129 



signal is coming in. The dotted line shows the aver- 
age current. 

Suppose that we take the same tube and use a 
B-battery of lower voltage. The characteristic will 
have the same shape but there will not be as much 
current unless the grid helps, so that the character- 
istic will be like that of Fig. 56. This characteristic 
crosses the axis of zero volts at a smaller number 
of mil-amperes than does the other because the B- 




Ciz/r/^jEr/vr 






batteries can't pull as hard as they did in the other 
case. 

You can see the result. When the grid becomes 
positive it helps and increases the plate current. 
When it becomes negative it opposes and decreases 
the plate current. But the increase just balances the 
decrease, so that on the average the current is un- 
changed, as shown by the dotted line. 

On the other hand, if we use a still smaller volt- 
age of B-battery we get a characteristic which shows 
a still smaller current when the grid is at zero po- 



130 LETTERS OF A RADIO-ENGINEER 

tential. For this case, as shown in Fig. 57, the plate 
current is larger on the average when there is an 
incoming signal. 

If we want to know whether or not there is any- 
incoming signal we will not nse the tube in the sec- 
ond condition, that of Fig. 56, because it won't tell 
us anything. On the other hand why use the tube 
under the first conditions where we need a large 
plate battery? If w^e can get the same result, that 
is an indication when the other station is signalling, 
by using a small battery let's do it that way for 
batteries cost money. For that reason we shall con- 
fine ourselves to the study of what takes place under 
the conditions of Fig. 57. 

We now know that when a signal is being sent 
by the distant station the current in the plate circuit 
of our audion at the receiving station is greater, on 
the average. We are ready to see what effect this 
has on the telephone receiver. And to do this re- 
quires a little study of how the telephone receiver 
works and why. 

I shall not stop now to tell you much about the 
telephone receiver for it deserves a whole letter all 
to itself. You know that a magnet attracts iron. 
Suppose you wind a coil of insulated wire around 

a bar magnet or put the 

^^^^^^rAr^ . „ c- magnet mside such a coil 




as in Fig. 58. Send a stream 

r/qSa^^ electrons through the 
turns of the coil — a steady 
stream such as comes from the battery shown in 



WHY WE USE A DETECTOR 



131 







H|i|iH 



ng^9 



the figure. The strength of the magnet is altered. 
For one direction of the electron stream through 
the coil the magnet is stronger. For the opposite 
direction of current the magnet will be weaker. 

Fig. 59 shows a simple design 
of telephone receiver. It is 
formed by a bar magnet, a coil 
about it through which a cur- 
rent can flow, and a thin disc 
of iron. The iron disc, or dia- 
phragm, is held at its edges 
so that it cannot move as a 
whole toward the magnet. The 
center can move, however, and so the diaphragm is 
bowed out in the form shown in the smaller sketch. 

Now connect a battery to the receiver winding 
and allow a steady stream of electrons to flow. The 
magnet will be either strengthened or weakened. 
Suppose the stream of electrons is in the direction 
to make it stronger — I'll give you the rule later. 
Then the diaphragm is bowed out still more. If we 
open the battery circuit and so stop the stream of 
electrons the diaphragm will fly back to its original 
position, for it is elastic. The effect is very much 
that of pushing in the bottom of a tin pan and letting 
it fly back when you remove your hand. 

Next reverse the battery. The magnet does not 
pull as hard as it would if there were no current. 
The diaphragm is therefore not bowed out so much. 

Suppose that instead of reversing the current by 
reversing the battery we arrange to send an alter- 



132 LETTERS OF A RADIO-ENGINEER 

nating current through the coil. That will have the 
same effect. For one direction of current flow, the 
diaphragm is attracted still more by the magnet 
but for the other direction it is not attracted as 
much. The result is that the center of the diaphragm 
moves back and forth during one complete cycle of 
the alternating current in the coil. 

The diaphragm vibrates back and forth in tune 
with the alternating current in the receiver winding. 
As it moves away from the magnet it pushes ahead 
of it the neighboring molecules of air. These mole- 
cules then crowd and push the molecules of air which 
are just a little further away from the diaphragm. 
These in turn push against those beyond them and so 
a push or shove is sent out by the diaphragm from 
molecule to molecule until perhaps it reaches your 
ear. When the molecules of air next your ear receive 
the push they in turn push against your eardrum. 

In the meantime what has happened 1 The current 
in the telephone receiver has reversed its direction. 
The diaphragm is now pulled toward the magnet 
and the adjacent molecules of air have even more 
room than they had before. So they stop crowding 
each other and follow the diaphragm in the other 
direction. The molecules of air just beyond these, 
on the way toward your ear, need crowd no longer 
and they also move back. Of course, they go even 
farther than their old positions for there is now 
more room on the other side. That same thing hap- 
pens all along the line until the air molecules next 
your ear start back and give your eardrum a chance 



WHY WE USE A DETECTOR 133 

to expand outward. As they move away they make 
a little vacuum there and the eardrum puffs out. 

That goes on over and over again just as often as 
the alternating current passes through one cycle of 
values. And you, unless you are thinking partic- 
ularly of the scientific explanations, say that you 
^'hear a musical note.'' As a matter of fact if we 
increase the frequency of the alternating current you 
will say that the ^' pitch" of the note has been in- 
creased or that you hear a note higher in the musical 
scale. 

If we started with a very low-frequency alternat- 
ing current, say one of fifteen or twenty cycles per 
second, you wouldn't say you heard a note at all. 
You would hear a sort of a rumble. If we should 
gradually increase the frequency of the alternating 
current you would find that about sixty or perhaps 
a hundred cycles a second would give you the impres- 
sion of a musical note. As the frequency is made still 
larger you have merely the impression of a higher- 
pitched note until we get up into the thousands of 
cycles a second. Then, perhaps about twenty- thou- 
sand cycles a second, you find you hear only a little 
sound like wind or like steam escaping slowly from a 
jet or through a leak. A few thousand cycles more 
each second and you don't hear anything at all. 

You know that for radio-transmitting stations we 
use audion oscillators which are producing alternat- 
ing currents with frequencies of several hundred- 
thousand cycles per second. It certainly wouldn't 
do any good to connect a telephone receiver in the 



134 LETTERS OF A RADIO-ENGINEER 




antenna circuit at the receiving station as in Fig. 60. 
We couldn't hear so high pitched a note. 

Even if we could, there are several reasons why 
the telephone receiver wouldn't work at such high 

frequencies. The first is 
that the diaphragm can't be 
moved so fast. It has some 
inertia, you know, that is, 
some unwillingness to get 
started. If you try to start 
_ it in one direction and, be- 

r/c^ou £Q^g y^^ really get it going, 
change your mind and try to make it go in the 
other direction, it simply isn't going to go at all. 
So even if there is an alternating current in the 
coil around the magnet there will not be any corre- 
sponding vibration of the diaphragm if the frequency 
is very high, certainly not if it is above about 20,000 
cycles a second. 

The other reason is that there will only be a very 
feeble current in the coil anyway, no matter what you 
do, if the frequency is high. You remember that the 
electrons in a coil are sort of banded together and 
each has an effect on all the others which can move 
in parallel paths. The result is that they have a 
great unwillingness to get started and an equal un- 
willingness to stop. Their unwillingness is much 
more than if the wire was long and straight. It is 
also made very much greater by the presence of the 
iron core. An alternating e. m. f . of high frequency 
hardly gets the electrons started at all before it's 



WHY WE USE A DETECTOR 135 

time to get them going in the opposite direction. 
There is very little movement to the electrons and 
hence only a very small current in the coil if the fre- 
quency is high. 

If you want a rule for it you can remember that 
the higher the frequency of an alternating e. m. f. 
the smaller the electron stream which it can set oscil- 
lating in a given coil. Of course, we might make 
the e. m. f. stronger, that is pull and shove the elec- 
trons harder, but unless the coil has a very small 
inductance or unless the frequency is very low we 
should have to use an e. m. f. of enormous strength 
to get any appreciable current. 

Condensers are just the other way in their action. 
If there is a condenser in a circuit, where an alter- 
nating e. m. f . is active, there is lots of trouble if the 
frequency is low. If, however, the frequency is high 
the same-sized current can be maintained by a 
smaller e. m. f . than if the frequency is low. You see, 
when the frequency is high the electrons hardly get 
into the waiting-room of the condenser before it is 
time for them to turn around and go toward the other 
room. Unless there is a large current, there are not 
enough electrons crowded together in the waiting- 
room to push back very hard on the next one to be 
sent along by the e. m. f. Because the electrons do 
not push back very hard a small e. m. f. can drive 
them back and forth. 

Ordinarily we say that a condenser impedes an 
alternating current less and less the higher is the 
frequency of the current. And as to inductances, we 
say that an inductance impedes an alternating cur- 



136 LETTERS OF A RADIO-ENGINEER 

rent more and more the higher is the frequency. 

Now we are ready to study the receiving circuit 
of Fig. 54. I showed you in Fig. 57 how the current 
through the tube will vary as time goes on. It in- 
creases and decreases with the frequency of the 
current in the antenna of the distant transmitting 
station. We have a picture, or graph as we say, of 
how this plate current varies. It will be necessary 
to study that carefully and to resolve it into its 
components, that is to separate it into parts, which 
added together again will give the whole. To show 
you what I mean I am going to treat first a very 
simple case involving money. 

Suppose a boy was started by his father with 50 
cents of spending money. He spends that and runs 
50 cents in debt. The next day his father gives him 
a dollar. Half of this he has to spend to pay up his 
yesterday's indebtedness. This he does at once and 
that leaves him 50 cents ahead. But again he buys 
something for a dollar and so runs 50 cents in debt. 
Day after day this cycle is repeated. We can show 
what happens by the curve of Fig. 61a. 

A^o^E^ y^foc^^r On the other hand, sup- 
pose he already had 60 
cents which he was saving 
for some special purpose. 
//-/DEBT This he doesn't touch, pre- 
F^/Q S/a, f erring to run into debt 

each day and to pay up the next, as shown in 
Fig. 61a. Then we would represent the story of this 
60 cents by the graph of Fig. 61b. 




WHY WE USE A DETECTOR 137 

Now suppose that instead of going in debt each 
day he uses part of this 60 cents. Each day after 
the first his father gives him l ~ 

a dollar, just as beiore. lie ___~ 

starts then with 60 cents as ///^^qt^^^^ 

shown in Fig. 61c, increases /Hq ^/^ 

in wealth to $1.10, then spends $1.00, bringing his 

funds down to 10 cents. Then he receives $1.00 

from his father and the process is repeated 

cyclically. 

If you saw the graph of Fig. 61c you would be 
able to say that, whatever he actually did, the effect 
was the same as if he had two pockets, in one of 
I^A/o , ^ ^ which he kept 60 cents all 

.60 J\ A JSj/^^Z^^ ^^^ ^™^ ^^ shown in Fig. 
* ' y Y M^^^^ 61b. In his other pocket 

'o-^^-^ — ^ — he either had money or he 

/v (7 ^/ c was in debt as shown in 

Fig. 61a. If you did that you would be resolving 
the money changes of Fig. 61c into the two com- 
ponents of Figs. 61a and b. 

That is what I want you to do with the curve of 
Fig. 57 which I am reproducing here, redrawn as 
Fig. 62a. You see it is really 
the result of adding together the 
two curves of Figs. 62b and c, 
which are shown on the follow- 
ing page. nQe2cL 

We can think, therefore, of the current in the plate 
circuit as if it were two currents added together, that 
is, two electron streams passing through the same 




1B8 LETTERS OF A RADIO-ENGINEER 

wire. One stream is steady and the other alternates. 

Now look again at the diagram of our receiving 
set which I am reproducing as Fig. 63. When the 

— signal is incoming there flow in 

_the plate circuit two streams of 

F'/G S2 £- electrons, one steady and of a 
value in mil-amperes corresponding to that of the 
graph in Fig. 62b, and the other alternating as 
shown in Fig. 62c. 

The steady stream of electrons will have no more 
difficulty in getting through the coiled wire of the 
receiver than it would through the same amount of 
straight wire. On the other hand it cannot pass 
the gap of the condenser. 

The alternating-current component can't get along 
in the coil because its frequency is so high that the 
coil impedes the motion of the electrons so much as 
practically to stop them. On the other hand these 
electrons can easily run into the waiting-room of- 
fered by the condenser and then run out again as 
soon as it is time. 

When the current in the plate circuit is large all 
the electrons which aren't needed for the steady 
stream through the telephone 
receiver run into one plate of 
the condenser. Of course, at 
that same instant an equal 
number leave the other plate F'/C^ &^C 

and start off toward the B-battery and the filament. 
An instant later, when the current in the plate cir- 
cuit is small, electrons start to come out of the 




WHY WE USE A DETECTOR 



139 







plate and to join the stream through the receiver 
so that this stream is kept steady. 

This steady stream of electrons, which is passing 
through the receiver winding, 
is larger than it would be if 
there was no incoming radio 
signal. The result is a stronger 
pull on the diaphragm of the 
receiver. The moment the sig- 
nal starts this diaphragm is 
pulled over toward the magnet 
and it stays pulled over as long as the signal lasts. 
When the signal ceases it flies back. We would 
hear then a click when the signal started and an- 
other when it stopped. 

If we wanted to distinguish dots from dashes this 
wouldn't be at all satisfactory. So in the next letter 
I'll show you what sort of changes we can make in 
the apparatus. To understand what effect these 
changes will have you need, however, to understand 
pretty well most of this letter. 



LETTER 15 
RADIO-TELEPHONY 

Deab Lad: 

Before we start on the important subject matter 
of this letter let ns make a short review of the preced- 
ing two letters. 

An oscillating audion at the transmitting station 
produces an effect on the plate current of the detec- 
tor audion at the receiving station. There is im- 
pressed upon the grid of the detector an alternating 
e. m. f. which has the same frequency as the alter- 
nating current which is being produced at the send- 
ing station. While this e. m. f. is active, and of 
course it is active only while the sending key is held 
down, there is more current through the winding of 
the telephone receiver and its diaphragm is conse- 
quently pulled closer to its magnet. 

What will happen if the e. m. f. which is active 
on the grid of the detector is made stronger or 
weaker ? The pull on the receiver diaphragm will be 
stronger or weaker and the diaphragm will have to 
move accordingly. If the pull is weaker the elasticity 
of the iron will move the diaphragm away from the 
magnet, but if the pull is stronger the diaphragm ^\all 
be moved toward the magnet. 

Every time the diaphragm moves it affects the air 
in the immediate neighborhood of itself and that air 

140 



RADIO-TELEPHONY 



14*1 



in turn affects the air farther away and so the ear 
of the listener. Therefore if there are changes in 
the intensity or strength of the incoming signal 
there are going to be corresponding motions of the 
receiver diaphragm. And something to listen, too, 
if these changes are frequent enough but not so fre- 
quent that the receiver diaphragm has difficulty in 
following them. 

There are many ways of affecting the strength of 
the incoming signal. Suppose, for example, that we 
arrange to decrease the current in the antenna of 
the transmitting station. That will mean a weaker 
signal and a smaller increase in current through 
the winding of the telephone receiver at the other 
station. On the other hand if the signal strength 
is increased there is more current in this winding. 

Suppose we connect a fine wire in the antenna 
circuit as in Fig. 64 and have a sliding contact as 
shown. Suppose that when we depress the switch 
in the oscillator circuit and 
so start the oscillations that 
the sliding contact is at o 
as shown. Corresponding to 
that strength of signal there 
is a certain value of current 
through the receiver winding 
at the other station. Now 
let us move the slider, first 
to a and then back to h and 
so on, back and forth. You see what will happen. 
We alternately make the current in the antenna 




in 



g=H|iiip 
'n_-^i,l,|,p 



kV//P£* 



r/oe^ 



142 LETTERS OF A RADIO-ENGINEER 







larger and smaller than it originally was. When 
the slider is at h there is more of the fine wire in 
series with the antenna, hence more resistance to 

the oscillations of the 

electrons, and hence 

a smaller oscillating 

stream of electrons. 

That means a weaker 

/v C7 ^5 outgoing signal. When 

the slider is at a there is less resistance in the 

antenna circuit and a larger alternating current. 

A picture of what happens would be like that of 
Fig. 65. The signal varies in intensity, therefore, 

tvHfCH you ^p8:nK 

^/^/RLL Df/RPHR/^Qf^ 
TO RSTRiri Q/^/=ir/uLAR 

c/^RBor/ QurT-or/ 

£-L£'CrROO£ 









becoming larger and smaller alternately. That 
means the voltage impressed on the grid of the detec- 
tor is alternately larger and smaller. And hence 
the stream of electrons through the winding of the 
telephone receiver is alternately larger and smaller. 






RADIO-TELEPHONY 143 

And that means that the diaphragm moves back and 
forth in just the time it takes to move the slider 
back and forth. 

Instead of the slider we might use a little cup al- 
most full of grains of carbon. The carbon grains 
lie between two flat discs of carbon. One of these 
discs is held fixed. The 
other is connected to the 
center of a thin diaphragm 
of steel and moves back ' ^oF-D/^^H/=?/=jom 

and forth as this dia- 
phragm is moved. The whole thing makes a tele- 
phone transmitter such as you have often talked to. 
Wires connect to the carbon discs as shown in 
Fig: ^^. A stream of electrons can flow through the 
wires and from grain to grain through the *^ carbon 
button," as we call it. The electrons have less diffi- 
culty if the grains are compressed, that is the button 
then offers less resistance to the flow of current. If 
the diaphragm moves back, allowing the grains to 
have more room, the electron stream is smaller and 
we say the button is offering more resistance to the 
current. 

You can see what happens. Suppose some one 

talks into the transmitter 
and makes its diaphragm 
go back and forth as shown 
in Fig. 67a. Then the cur- 
^/yr£-/y///^ cunREriy-r i^ent in the antenna varies, 
F'/Q S7-(> being greater or less, de- 

pending upon whether the button offers less or 




144 LETTERS OF A RADIO-ENGINEER 

more resistance. The corresponding variations in 
the antenna current are shown in Fig. 67b. 

In the antenna at the receiving station there are 
corresponding variations in the strength of the sig- 
nal and hence corresponding variations in the 
strength of the current through the telephone re- 
ceiver. I shall show graphically what happens in 
Fig. 68. You see that the telephone receiver 
diaphragm does just the same motions as does the 
transmitter diaphragm. That means that the mole- 
cules of air near the receiver diaphragm are going 
through just the same kind of motions as are those 
near the transmitter diaphragm. When these air 
molecules affect your eardrum you hear just what 
you would have heard if you had been right there be- 
side the transmitter. 

That's one way of making a radio-telephone. It 
is not a very efficient method but it has been used in 
the past. Before we look at any of the more recent 
methods we can draw some general ideas from this 
method and learn some words that are used almost 
always in speaking of radio-telephones. 

In any system of radio-telephony you will al- 
ways find that there is produced at the transmit- 
ting station a high-frequency alternating current and 
that this current flows in a tuned circuit one part of 
which is the condenser formed by the antenna and 
the ground (or something which acts like a ground). 
This high-frequency current, or radio-current, as we 
usually say, is varied in its strength. It is varied in 
conformity with the human voice. If the human 



RADIO-TELEPHONY 



145 



voice speaking into the transmitter is low pitched 
there are slow variations in the intensity of the radio 
current. If the voice is high pitched there are more 
rapid variations in the strength of the radio-fre- 
qnency current. That is why we say the radio-cur- 
rent is ''modulated'' by the human voice. 








/DOTTS^D L/ME- IS 



ng 63 



The signal which radiates out from the transmit- 
ting antenna carries all the little variations in pitch 
and loudness of the human voice. When this sig- 
nal reaches the distant antenna, it establishes in that 
antenna circuit a current of high frequency which 
has just the same variations as did the current in 
the antenna at the sending station. The human voice 
isn't there. It is not transmitted. It did its work 
at the sending station by modulating the radio-signal, 



146 LETTERS OF A RADIO-ENGINEER 

^^ modulating the carrier current," as we sometimes 
say. But there is speech significance hidden in the 
variations in strength of the received signal. 

If a telephone-receiver diaphragm can be made to 
Tdbrate in accordance with the variations in signal 
intensity then the air adjacent to that diaphragm will 
be set into vibration and these vibrations will be jnst 
like those which the human voice set up in the air 
molecules near the mouth of the speaker. All the 
different systems of receiving radio-telephone sig- 
nals are merely different methods of getting a cur- 
rent which will affect the telephone receiver in con- 
formity with the variations in signal strength. 
Getting such a current is called ^^ detecting." 
There are many different kinds of detectors 
but the vacuum tube is much to be preferred. 

The cheapest detector, but not the most sensitive, 
is the crystal. If you understand how the audion 
works as a detector you will have no difficulty in 
understanding the crystal detector. 

The crystal detector consists of some mineral crys- 
tal and a fine-wire point, usually platinum. Crystals 
are peculiar things. Like everything else they are 
made of molecules and these molecules of atoms. 
The atoms are made of electrons grouped around 
nuclei which, in turn, are formed by close group- 
ings of protons and electrons. The great difference 
between crystals and substances which are not crys- 
talline, that is, substances which don't have a special 
natural shape, is this : In crystals the molecules and 
atoms are all arranged in some orderly manner. 



RADIO-TELEPHONY 147 

In other substances, substances without special form, 
amorphous substances, as we call them, the molecules 
are just grouped together in a haphazard way. 

For some crystals we know very closely indeed 
how their molecules or rather their individual atoms 
are arranged. Sometime you may wish to read how 
this was found out by the use of X-rays. ^ Take 



the crystal of common salt for example. That is 
well known. Each molecule of salt is formed by an 
atom of sodium and one of chlorine. In a crystal 
of salt the molecules are grouped together so that 
a sodium atom always has chlorine atoms on every 
side of it, and the other way around, of course. 

Suppose you took a lot of wood dumb-bells and 
painted one of the balls of each dumb-bell black to 
stand for a sodium atom, leaving the other unpainted 

1 Cf . "Within the Atom/' Chapter X. 



148 LETTERS OF A RADIO-ENGINEER 



to stand for a chlorine atom. Now try to pile them 
up so that above and below each black ball, to the 
right and left of it, and also in front and behind it, 
there shall be a white ball. The pile which you would 
probably get would look like that of Fig. 69. I have 
omitted the gripping part of each dumbell because I 
don't believe it is there. In my picture each circle 
represents the nucleus of an atom. I haven't at- 
tempted to show the planetary electrons. Other 
crystals have more complex arrangements for piling 
up their molecules. 

Now suppose we put two different kinds of sub- 
stances close together, that is, make contact between 
them. How their electrons will behave v/ill depend 
entirely upon what the atoms are and how they are 
piled up. Some very curious effects can be obtained. 
The one which interests us at present is that across 
the contact points of some combinations of sub- 
stances it is easier to get a stream of electrons to 
flow one way than the other. The contact doesn't 

have the same resistance in 
the two directions. Usually 
also the resistance depends 
upon what voltage we are 
applying to force the elec- 

f r — tron stream across the point 

(/j "^orrecTOH of contact. 

i 1 ] The one way to find out is 

to take the voltage-current 
To do so we use 




characteristic of the combination. 

the same general method as we did for the audion. 



RADIO-TELEPHONY 



149 



J/OLTf^£ji^ 






F-fQ 7/ 



And when we get through we plot another curve 
and call it, for example, a '^platinum-galena char- 
acteristic.'' Fig. 70 shows the set-up for making 
the measurements. There is a 
group of batteries arranged so 
that we can vary the e. m. f . 
applied across the contact point 
of the crystal and platinum. 
A voltmeter shows the value of 
this e. m. f . and an ammeter 
tells the strength of the electron stream. Each time 
we move the slider we get a new pair of values for 
volts and amperes. As a matter of fact we don't 
get amperes or even mil-amperes ; we get millionths 
of an ampere or '^ micro-amperes," as we say. We 
can plot the pairs of values which we measure and 
make a curve like that of Fig. 71. 

When the voltage across the contact is reversed, 
of course, the current reverses. Part of the curve 
looks something like the lower part of an audion 
characteristic. 

Now connect this crystal in a receiving circuit as 
in Fig. 72. We use an antenna just as we did for 
the audion and we tune the antenna circuit to the fre- 
quency of the incoming signal. 
The receiving circuit is coupled 
to the antenna circuit and is 
tuned to the same frequency. 
Whatever voltage there may be 
across the condenser of this circuit is applied to the 
crystal detector. We haven't put the telephone 








150 LETTERS OF A RADIO-ENGINEER 

receiver in the circuit yet. I want to wait until you 
have seen what the crystal does when an alternating 
voltage is applied to it. 

We can draw a familiar form of sketch as in Fig. 
73 to show how the current in 
'A A A A ^^^ crystal varies. You see that 
V v/\/V"> there flows through the crystal 
?c/m^/$^f a current very much like that of 
,3 /r/co^f/^q Fig. 62a. And you know that 

'^^ 7>^ such a current is really equiva- 

lent to two electron streams, one steady and the 
other alternating. The crystal detector gives us 
much the same sort of a current as does the vacuum 
tube detector of Fig. 54. The current isn't any- 
where near as large, however, for it is micro- 
amperes instead of mil-amperes. 

Our crystal detector produces the same results so 
far as giving us a steady component of current to 
send through a telephone receiver. So we can con- 
nect a receiver in series with the crystal as shown 
in Fig. 74. Because the receiver would offer a large 
impedance to the high-frequency current, that is, se- 
riously impede and so reduce the high-frequency 
current, we connect a condenser around the receiver. 

There is a simple crystal 
detector circuit. If the sig- 
nal intensity varies then the 

current which passes through r£:LEPHOf>/£^ 

the receiver will vary. If ^^<^ 7"^ ^^^o^^r 
these variations are caused by a human voice at the 
sending station the crystal will permit one to hear 




RADIO-TELEPHONY 151 

from the telephone receiver what the speaker is 
saying. That is just what the audion detector does 
very many times better. 

In the letter on how to experiment you'll find 
details as to the construction of a crystal-detector 
set. Excellent instructions for an inexpensive set 
are contained in Bull. No. 120 of the Bureau of 
Standards. A copy can be obtained by sending ten 
cents to the Commissioner of Public Documents, 
Washington, D. C. 



LETTER 16 
THE HUMAN VOICE 

Deae Sm: 

The radio-telephone does not transmit the human 
voice. It reproduces near the ears of the listener 
similar motions of the air molecules and hence causes 
in the ears of the listener the same sensations of 
sound as if he were listening directly to the speaker. 




/^^ 75 

This reproduction takes place almost instantaneously 
so great is the speed with which the electrical effects 
travel outward from the sending antenna. If you 
wish to understand radio-telephony you must know 
something of the mechanism by which the voice is 
produced and something of the peculiar or charac- 
teristic properties of voice sounds. 

The human voice is produced by a sort of organ 
pipe. Imagine a long pipe connected at one end to 
a pair of fire-bellows, and closed at the other end by 
two stretched sheets of rubber. Fig. 75 is a sketch of 

152 



THE HUMAN VOICE 



153 




wliat I mean. Corresponding to the bellows there is 
the human diaphragm, the muscular membrane sep- 
arating the thorax and abdomen, which expands or 
contracts as one breathes. Corresponding to the 
pipe is the windpipe. Corresponding to the 
two stretched pieces of rubber 
are the vocal cords, L and R, 
shown in cross section in Fig. 
77. They are part of the 
larynx and do not show in 
Fig. 76 (PI. viii) which shows ^ 
the wind pipe and an outside 
view of the larynx. 

When the sides of the bel- 
lows are squeezed together 
the air molecules within are 
crowded closer together and ^^ Q 77 

the air is compressed. The greater the compres- 
sion the greater, of course, is the pressure with 
which the enclosed air seeks to escape. That it can 
do only by lifting up, that is by blowing out, the 
two elastic strips which close the end of the pipe. 

The air pressure, therefore, rises until it is suffi- 
cient to push aside the elastic membranes or vocal 
cords and thus to permit some of the air to escape. 
It doesn't force the membranes far apart, just 
enough to let some air out. But the moment some 
air has escaped there isn't so much inside and the 
pressure is reduced just as in the case of an auto- 
mobile tire from which you let the air escape. What 
is the result? The membranes fly back again and 



feij 



154 LETTERS OF A RADIO-ENGINEER 

close the opening of the pipe. What got out, then, 
was just a little puff of air. 

The bellows are working all the while, however, 
and so the space available for the remaining air soon 
again becomes so crowded with air molecules that 
the pressure is again sufficient to open the mem- 
branes. Another puff of air escapes. 

This happens over and over again while one is 
speaking or singing. Hundreds of times a second 
the vocal cords vibrate back and forth. The fre- 
quency with which they do so determines the note 
or pitch of the speaker's voice. 

What determines the significance of the sounds 
which he utters f This is a most interesting question 
and one deserving of much more time than I propose 
to devote to it. To give you enough of an answer 
for your study of radio-telephony I am going to tell 
you first about vibrating strings for they are easier 
to picture than membranes like the vocal cords. 

Suppose you have a stretched string, a piece of 
rubber band or a wire will do. You pluck it, that 
is pull it to one side. When you let go it flies back. 
Because it has inertia^ it doesn't stop when it gets 
to its old position but goes on through until it bows 
out almost as far on the other side. 

It took some work to pluck this string, not much 
perhaps ; but all the work which you did in deform- 
ing it, goes to the strinof and becomes its energy, 
its ability to do work. This work it does in pushing 
the air molecules ahead of it as it vibrates. In 

iCf. Chap. VI of "The Realities of Modern Science." 




Fig 79 




ri<s &0 



Pl. VII. — Photographs of Vibrating Strings. 



! 



THE HUMAN VOICE 155 

this way it uses up its energy and so finally comes 
again to rest. Its vibrations ^'damp out," as we 
say, that is die down. Each swing carries it a smaller 
distance away from its original position. We say 
that the ' ' amplitude, ' ' meaning the size, of its vibra- 
tion decreases. The frequency does not. It takes 
just as long for a small-sized vibration as for the 
larger. Of course, for the vibration of large ampli- 
tude the string must move faster but it has to move 
farther so that the time required for a vibration is 
not changed. 

First the string crowds against each other the air 
molecules which are in its way and so leads to crowd- 
ing further away, just as fast as these molecules can 
pass along the shove they are receiving. That takes 
place at the rate of about 1100 feet a second. When 
the string swings back it pushes away the molecules 
which are behind it and so lets those that were 
being crowded follow it. You know that they will. 
Air molecules will always go where there is the least 
crowding. Following the shove, therefore, there is 
a chance for the molecules to move back and even to 
occupy more room than they had originally. 

The news of this travels out from the string just 
as fast as did the news of the crowding. As fast as 
molecules are able they move back and so make more 
room for their neighbors who are farther away ; and 
these in turn move back. 

Do you want a picture of it? Imagine a great 
crowd of people and at the center some one with 
authority. The crowd is the molecules of air and the 



156 LETTERS OF A RADIO-ENGINEER 

one with authority is one of the molecules of the 
string which has energy. Whatever this molecule of 
the string says is repeated by each member of the 
crowd to his neighbor next farther away. First the 
string says: *'Go back" and each molecule acts as 
soon as he gets the word. And then the string says : 
''Come on" and each molecule of air obeys as soon 
as the command reaches him. Over and over this 
happens, as many times a second as the string makes 
complete vibrations. 

If we should make a picture of the various posi- 
tions of one of these air molecules much as we 
pictured ^'BroTsmie" in Letter 9 it would appear as in 
Fig. 78a where the central line represents the ordi- 
nary position of the molecule. 

That's exactly the picture also of the successive 
positions of an electron in a circuit which is ' ' carry- 
ing an alternating current." First it moves in one di- 
rection along the mre and 
then back in the opposite di- 
rection. The electron next to 
it does the same thing almost 
immediately for it does not 
take anywhere near as long 
for such an effect to pass ^^^^^^^^^^^^^ 
through a crowd of electrons. '^'^^0%''^^% ^^^"'^^ 

If we make the string vi- or^er ^eovs: 

brate twice as fast, that is, ^ r<^ 

have twice the frequency, the story of an adjacent 
particle of air will be as in Fig. 78b. Unless we 
tighten the string, however, we can't make it vibrate 




THE HUMAN VOICE 157 

as a whole and do it twice as fast. We can make 
it vibrate in two parts or even in more parts, as 
shown in Fig. 79 of PI. VII. When it vibrates as a 
whole, its frequency is the lowest possible, the fun- 
damental frequency as we say. When it vibrates in 
two parts each part of the string makes twice as 
many vibrations each second. So do the adjacent 
molecules of air and so does the eardrum of a lis- 
tener. 

The result is that the listener hears a note of twice 
the frequency that he did when the string was vi- 
brating as a whole. He says he hears the ^ ^octave'' 
of the note he heard first. If the string vibrates in 
three parts and gives a note of three times the fre- 
quency the listener hears a note two octaves above 
the '* fundamental note'' of which the string is 
capable. 

It is entirely possible, however, for a string to vi- 
brate simultaneously in a number of ways and so 
to give not only its fundamental note but several 
others at the same time. The photographs ^ of Fig. 
80 of PI. VII illustrate this possibility. 

What happens then to the molecules of air which 
are adjacent to the vibrating string? They must 
perform quite complex vibrations for they are called 
upon to move back and forth just as if there were 
several strings all trying to push them with different 
frequencies of vibration. Look again at the pictures 
of Fig. 80 and see that each might just as well be 

1 My thanks are due to Professor D. C. Miller and to the Mac- 
millan Company for permission to reproduce Figs. 79 to 83 inclu- 
sive from that interesting book, "The Science of Musical Sounds."' 



158 LETTERS OF A RADIO-ENGINEER 

the picture of several strings placed close together, 
each vibrating in a different way. Each of the 
strings has a different frequency of vibration and 
a different maximum amplitude, that is, greatest size 
of swing away from its straight position. 




Suppose instead of a single string acting upon 
the adjacent molecules we had three strings. Sup- 
pose the first would make a nearby molecule move as 
in Fig. 81A, the second as in Fig. 81B, and the 
third as in Fig. 81C. It is quite evident that the mole- 
cule can satisfy all three if it will vibrate as in Fig. 
BID. 



THE HUMAN VOICE 159 

Now take it the other way around. Suppose we 
had a picture of the motion of a molecule and that it 
was not simple like those shown in Fig. 78 but was 
complex like that of Fig. 81D. We could say that 
this complex motion was made up of three parts, that 
is, had three component simple motions, each repre- 
sented by one of the three other graphs of Fig. 81. 
That means we can resolve any complex vibratory 
motion into component motions which are simple. 

It means more than that. It means that the vibrat- 
ing string which makes the neighboring molecules 
of air behave as shown in Fig. 81D is really acting 
like three strings and is producing simultaneously 
three pure musical notes. 

Now suppose we had two different strings, say a 
piano string in the piano and a violin string on its 
proper mounting. Suppose we played both instru- 
ments and some musician told us they were in tune. 
What would he mean? He would mean that both 
strings vibrated with the same fundamental fre- 
quency. 

They differ, however, in the other notes which they 
produce at the same time that they produce their fun- 
damental notes. That is, they differ in the frequen- 
cies and amplitudes of these other component vibra- 
tions or *' overtones" which are going on at the same 
time as their fundamental vibrations. It is this dif- 
ference which lets us tell at once which instrument is 
being played. 

That brings us to the main idea about musical 
sounds and about human speech. The pitch of any 



160 LETTERS OF A RADIO-ENGINEER 

complex sound is the pitch of its fundamental or low- 
est sound; but the character of the complex sound 
depends upon all the overtones or '* harmonics'' 
which are being produced and upon their relative fre- 
quencies and amplitudes. 




The organ pipe which ends in the larynx produces 
a very complex sound. I can't show you how com- 
plex but I'll show you in Fig. 82 the complicated 
motion of an air molecule which is vibrating as the 
result of being near an organ pipe. (Organ pipes 
differ — this is only one case.) You can see that 



THE HUMAN VOICE 161 

there are a large number of pure notes of various 
intensities, that is, strengths, which go to make up 
the sound which a listener to this organ pipe would 
hear. The note from the human pipe is much more 
complex. 

When one speaks there are little puffs of air escap- 
ing from his larynx. The vocal cords vibrate as I 
explained. And the molecules of air near the larynx 
are set into very complex vibrations. These trans- 
mit their vibrations to other molecules until those in 
the mouth are reached. In the mouth, however, some- 
thing very important happens. 

Did you ever sing or howl down a rain barrel or 
into a long pipe or hallway and hear the sound? 
It sounds just about the same no matter who does 
it. The reason is that the long column of air in the 
pipe or barrel is set into vibration and vibrates ac- 
cording to its own ideas of how fast to do it. It has 
a ^^ natural frequency" of its own. If in your voice 
there is a note of just that frequency it will respond 
beautifully. In fact it '^ resonates," or sings back, 
when it hears this note. 

The net result is that it emphasizes this note so 
much that you don^t hear any of the other compon- 
ent notes of your voice — all you hear is the rain 
barrel. We say it reinforces one of the component 
notes of your voice and makes it louder. 

That same thing happens in the mouth cavity of a 
speaker. The size and shape of the column of air 
in the mouth can be varied by the tongue and lip 
positions and so there are many different possibili- 



162 LETTERS OF A RADIO-ENGINEER 

ties of resonance. Depending on lip and tongne, dif- 
ferent frequencies of the complex sound which comes 
from the larynx are reinforced. You can see that for 
yourself from Fig. 83 which show^s the tongue posi- 
tions for three different vowel sounds. You can see 
also from Fig. 84, which shows the mouth positions 
for the different vowels, how the size and shape 
of the mouth cavity is changed to give different 
sounds. These figures are in PL VIII. 

The pitch of the note need not change as every 
singer knows. You can try that also for yourself 
by singing the vowel sound of ''ahh'' and then 
changing the shape of your mouth so as to give 
the sound ^'ah — aw — ow — ou.'' The pitch of the 
note will not change because the fundamental stays 
the same. The speech significance of the sound, how- 
ever, changes completely because the mouth cavity 
resonates to different ones of the higher notes which 
come from the larynx along with the fundamental 
note. 

Now you can see what is necessary for telephonic 
transmission. Each and every component note which 
enters into human speech must be transmitted and 
accurately reproduced by the receiver. More than 
that, all the proportions must be kept just the same 
as in the original spoken sound. We usually say 
that there must be reproduced in the air at the re- 
ceiver exactly the same ^'wave form" as is present 
in the air at the transmitter. If that isn't done the 
speech won't be natural and one cannot recognize 
voices although he may understand pretty well. If 



THE HUMAN VOICE 163 

there is too much ** distortion '' of the wave form, 
that is if the relative intensities of the component 
notes of the voice are too mnch altered, then there 
may even be a loss of intelligibility so that the lis- 
tener cannot understand what is being said. 

What particular notes are in the human voice de- 
pends partly on the person who is speaking. You 
know that the fundamental of a bass voice is lower 
than that of a soprano. Besides the fundamental, 
however, there are a lot of higher notes always pres- 
ent. This is particularly true when the spoken 
sound is a consonant, like *^s" or *^f '' or "v.'' The 
particular notes, which are present and are import- 
ant, depend upon what sound one is saying. 

Usually, however, we find that if we can transmit 
and reproduce exactly all the notes which lie between 
a frequency of about 200 cycles a second and one of 
about 2000 cycles a second the reproduced speech 
will be quite natural and very intelligible. For sing- 
ing and for transmitting instrumental music it is 
necessary to transmit and reproduce still higher 
notes. 

What you will have to look out for, therefore, in 
a receiving set is that it does not cut out some of 
the high notes which are necessary to give the sound 
its naturalness. You will also have to make sure 
that your apparatus does not distort, that is, does not 
receive and reproduce some notes or '^ voice fre- 
quencies'' more efficiently than it does some others 
which are equally necessary. For that reason when 
you buy a transformer or a telephone receiver it is 



164 LETTERS OF A RADIO-ENGINEER 

well to ask for a characteristic curve of the appar- 
atus which will show how the action varies as the fre- 
quency of the current is varied. The action or re- 
sponse should, of course, be practically the same at 
all the frequencies within the necessary part of the 
voice range. 



LETTEE 17 

GRID BATTERIES AND GRID CONDENSERS 
FOR DETECTORS 

Deak Son: 

You remember the audion characteristics which I 
used in Figs. 55, 56 and 57 of Letter 14 to show 
you how an incoming signal will affect the current in 
the plate circuit. Look again at these figures and 
you will see that these characteristics all had the 







same general shape but that they differed in their 
positions with reference to the ^^main streets" of 
^^zero volts" on the grid and *^zero mil-amperes" in 
the plate circuit. Changing the voltage of the B- 
battery in the plate circuit changed the position of 
the characteristic. We might say that changing the 
B-battery shifted the curve with reference to the 
axis of zero volts on the grid. 

165 



166 LETTERS OF A RADIO-ENGINEER 




In the case of the three characteristics which we 
are discussing the shift was made by changing the B- 
battery. Increasing B-voltage shifts characteristic 

to the left. It is possible, how- 
ever, to produce such a shift 
by using a C-battery, that is, 
a battery in the grid circuit, 
which makes the grid perma- 
nently negative (or positive, 
depending upon how it is con- 
r/QBJ '^^^•' nected). This battery either 
helps or hinders the plate battery, and because of 
the strategic position of the grid right near the fila- 
ment one volt applied to the grid produces as large 
an effect as would several volts in the plate battery. 
Usually, therefore, we arrange to shift the charac- 
teristic by using a C-battery. 

Suppose for example that we had an audion in the 
receiving circuit of Fig. 63 and that its characteris- 
tic under these conditions is 
given by Fig. 56. IVe redrawn 
the figures to save your turning 
back. The audion will not act 
as a detector because an incom- 
ing signal will not change the 
average value of the current in 
the plate circuit. If, however, 
we connect a C-battery so as to 
make the grid negative, we can '^ ^ 

shift this characteristic so that the incoming signal 
win be detected. We have only to make the grid 




GRID BATTERIES AND CONDENSERS 167 

sufficiently negative to reduce the plate current 
to the value shown by the line oa in Fig. 85. Then 
the signal will be detected because, while it makes 
the plate current alternately larger and smaller than 
this value oa^ it will result, on the average, in a higher 
value of the plate current. 

You see that what we have done is to arrange the 
point on the audion characteristic about which the 



/=7^c96 




tube is to work by properly choosing the value of the 
grid voltage Ec, 

There is an important method of using an audion 
for a detector where we arrange to have the grid 
voltage change steadily, getting more and more neg- 
ative all the time the signal is coming in. Before I 
tell how it is done I want to show you what will 
happen. 

Suppose we start with an audion detector, for 
which the characteristic is that of Fig. 56, but ar- 
ranged as in Fig. 86 to give the grid any potential 
which we wish. The batteries and slide wire re- 
sistance which are connected in the grid circuit are 
already familiar to you. 

When the slider is set as shown in Fig. SQ the grid 



168 LETTERS OF A RADIO-ENGINEER 

is at zero potential and we are at the point 1 of 
the characteristic shown in Fig. 87. Now imagine 
an incoming signal, as shown in that same figure, 
but suppose that as soon as the signal has stopped 



c PLPTs: 
CURREriT 




FfQ a? 



making the grid positive we shift the. slider a little 
so that the C-battery makes the grid slightl y nega- 
tive. We have shifted the point on the character- 
istic about which the tube is being worked by the 
incoming signal from point 1 to point 2. 

Every time the incoming signal makes one com- 
plete cycle of changes we shift the slider a little fur- 




GRID BATTERIES AND CONDENSERS 169 

ther and make the grid permanently more negative. 
You can see what happens. As the grid becomes 
more negative the current in the plate circuit de- 
creases on the average. Finally, of course, the grid 
will become so negative that the current in the plate 
circuit will be reduced to zero. Under these condi- 
tions an incoming signal finally makes a large change 
in the plate current and hence in the current through 
the telephone. 

The method of shifting a slider along, every time 
the incoming signal makes 
a complete cycle, is impos- ^^ J ^ c aI[ 
sible to accomplish by hand " 

if the frequency of the sig- ' L 
nal is high. It can be done ^ 

automatically, however, no matter how high the 
frequency if we use a condenser in the grid circuit 
as shown in Fig. 88. 

When the incoming signal starts a stream of elec- 
trons through the coil L of Fig.' 88 and draws them 
away from plate 1 of the condenser C it is also 
drawing electrons away from the 1 plate of the con- 
denser Cg which is in series with the grid. As elec- 
trons leave plate 1 of this condenser others rush 
away from the grid and enter plate 2. This means 
that the grid doesn't have its ordinary number of 
electrons and so is positive. 

If the grid is positive it will be pleased to get 
electrons; and it can do so at once, for there are 
lots of electrons streaming past it on their way to the 
plate. While the grid is positive, therefore, there 



\ 



170 LETTERS OF A RADIO-ENGINEER 

is a stream of electrons to it from the filament. Fig. 
89 shows this current. 

All this takes place during the first half -cycle of 
the incoming signal. During the next half -cycle elec- 
trons are sent into plate 1 of the condenser C and 
also into plate 1 of the grid condenser Cg. As elec- 
trons are forced into plate 1 
of the grid condenser those in 
plate 2 of that condenser have 
to leave and go back to the grid 
where they came from. That is 
all right, but while they were 
C^/RfD away the grid got some elec- 

trons from the filament to take 
FJqQS their places. The result is that 

the grid has now too many electrons, that is, it is 
negatively charged. 

An instant later the signal e.^ m. f . reverses and 
calls electrons away from plate 1 of the grid con- 
denser. Again electrons from the grid rush into 
plate 2 and again the grid is left without its proper 
number and so is positive. Again it receives elec- 
trons from the filament. The result is still more elec- 
trons in the part of the grid circuit which is formed 
by the grid, the plate 2 of the grid condenser and 
the connecting wire. These electrons can't get 
across the gap of the condenser Cg and they can't 
go back to the filament any other way. So there 
they are, traj)ped. Finally there are so many of 
these trapped electrons that the grid is so negative 
all the time as almost entirely to oppose the efforts of 
the plate to draw electrons away from the filament. 











CO 




GRID BATTERIES AND CONDENSERS 171 

Then the plate current is reduced practically to zero. 

That's the way to arrange an audion so that the 
incoming signal makes the largest possible change 
in plate current. We can tell if there is an incoming 
signal because it will ^' block" the tube, as we say. 
The plate-circuit current will be changed from its 
ordinary value to almost zero in the short time it 
takes for a few cycles of the incoming signal. 

We can detect one signal that way, but only one 
because the first signal makes the grid permanently 
negative and blocks the tube so that there isn't any 

/||2 




2 



CIpIiLL 



r/QSO 

current in the plate circuit and can't be any. If we 
want to put the tube in condition to receive another 
signal we must allow these electrons, which originally 
came from the filament, to get out of their trapped 
position and go back to the filament. 

To do so we connect a very fine wire between plates 
1 and 2 of the grid condenser. We call that wire a 
** grid-condenser leak" because it lets the electrons 
slip around past the gap. By using a very high 
resistance, we can make it so hard for the electrons 
to get around the gap that not many will do so 
while the signal is coming in. In that case we can 
leave the leak permanently across the condenser as 
shown in Fig. 90. Of course, the leak must offer so 
easy a path for the electrons that all the trapped 



172 LETTERS OF A RADIO-ENGINEER 

electrons can get home between one incoming signal 
and the next. 

One way of making a high resistance like this is 
to draw a heavy pencil line on a piece of paper, or 
better a line with India ink, that is ink made of fine 
ground particles of carbon. The leak should have 
a very high resistance, usually one or two million 
ohms if the condenser is about 0.002 microfarad. If 
it has a million ohms we say it has a '^megohm'' of 
resistance. 

This method of detecting with a leaky grid-con- 
denser and an audion is very efficient so far as 
telling the listener whether or not a signal is com- 
ing into his set. It is widely used in receiving radio- 
telephone s^nals although it is best adapted to re- 
ceiving the telegraph signals from a spark set. 

I don't propose to stop to tell you how a spark- 
set transmitter works. It is suf&cient to say that 
when the key is depressed the set sends out radio 
signals at the rate usually of 1000 signals a second. 
Every time a signal reaches the receiving station the 
current in the telephone receiver is sudden reduced ; 
and in the time between signals the lesfk across 
the grid condenser brings the tube back to a condition 
where it can receive the next signal. While the send- 
ing key is depressed the current in the receiver is 
decreasing and increasing once for every signal 
which is being transmitted. For each decrease and 
increase in current the diaphragm of the telephone 
receiver makes one vibration. What the listener then 
hears is a musical note with a frequency correspond- 



GRID BATTERIES AND CONDENSERS 173 



ing to that number of vibrations a second, that is, a 
note with a frequency of_one thousand cycles per sec- 
ond. He hears a note of frequency about that of two 
octaves above middle C on the piano. There are 






Q/^/D U^OLT/9Qi 




<f/?/o C6//?y?£'/yr 



/=»^/9 T^ CURf^EM T 




/mercer J i/£:f^ 

/v^ ^/ 

usually other notes present at the same time and the 
sound is not like that of any musical instrument. 

If the key is held down a long time for a dash 
the listener hears this note for a corresponding time. 
If it is depressed only about a third of that time. 



174 LETTERS OF A RADIO-ENGINEER 

so as to send a dot, the listener hears the note for 
a shorter time and interprets it to mean a dot. 

In Fig. 91 1 have drawn a sketch to show the e. m. f . 
which the signals from a spark set impress on the 
grid of a detector and to show how the plate current 
varies if there is a condenser and leak in the grid 
circuit. I have only shown three signals in succes- 
sion. If the operator sends at the rate of about 
twenty words a minute a dot is formed by about 
sixty of these signals in succession. 

The frequency of the alternations in one of the 
little signals will depend upon the wave length which 
the sending operator is using. If he uses the wave 
length of 600 meters, as ship stations do, he will send 
with a radio frequency of 500,000 cycles a second. 
Since the signals are at the rate of a thousand a 
second each one is made up of 500 complete cycles 
of the current in the antenna. It would be imprac- 
ticable therefore to show you a complete picture of 
the signal from a spark set. I have, however, let- 
tered the figure quite completely to cover what I have 
just told you. 

If the grid-condenser and its leak are so chosen 
as to work well for signals from a 500-cycle spark 
set they will also work well for the notes in human 
speech which are about 1000 cycles a second in fre- 
quency. The detecting circuit will not, however, 
work so well for the other notes which are in the hu- 
man voice and are necessary to speech. For ex- 
ample, if notes of about 2000 cycles a second are 
involved in the speech which is being transmitted, 



GRID BATTERIES AND CONDENSERS 175 

the leak across the condenser will not work fast 
enough. On the other hand, for the very lowest notes 
in the voice the leak will work too fast and such 
variations in the signal current will not be detected 
as efficiently as are those of 1000 cycles a second. 

You can see that there is always a little favoritism 
on the part of the grid-condenser detector. It 
doesn't exactly reproduce the variations in inten- 
sity of the radio signal which were made at the 
sending station. It distorts a little. As amateurs 
we usually forgive it that distortion because it is so 
efficient. It makes so large a change in the current 
through the telephone when it receives a signal that 
we can use it to receive much weaker signals, that 
is, signals from smaller or more distant sending sta- 
tions, than we can receive with the arrangement de- 
scribed in Letter 14. 



LETTER 18 

AMPLIFIERS AND THE REGENERATIVE 
CIRCUIT 

My Deak Receiver : 

There is one way of making an andion even more 
efficient as a detector than the method described in 
the last letter. And that is to make it talk to itself. 

Suppose we arrange a receiving circuit as in Fig. 
92. It is exactly like that of Fig. 90 of the previous 
letter except for the fact that the current in the plate 
circuit passes through a little coil, Lt, which is placed 
near the coil L and so can induce in it an e. m. f. 
which will correspond in intensity and wave form 
to the current in the plate circuit. 

If we should take out the grid condenser and its 
leak this circuit would be like that of Fig. 54 in Let- 
ter 13 which we used for a generator of high-fre- 
quency alternating currents. You remember how 
that circuit operates. A small effect in the grid 
circuit produces a large effect in the plate circuit. 
Because the plate circuit is coupled to the grid cir- 
cuit the grid is again affected and so there is a still 
larger effect in the plate circuit. And so on, until 
the current in the plate circuit is swinging from 
zero to its maximum possible value. 

What happens depends upon how closely the coils 
L and Lt are coupled, that is, upon how much the 

176 



THE REGENERATIVE CIRCUIT 



177 



changing current in one can affect the other. If 
the/" are turned at right angles to each other, so 
that there is no possible mutual effect we say there 
is ' ' zero coupling. ' ' 

Start with the coils at right angles to each other 
and turn Lt so as to bring its windings more and 
more parallel to those of L. If we want Lt to have 
a large effect on L its windings should be parallel 
and also in the same' direction just as they were in 
Fig. 54 of Letter 13 to which we just referred. As we 
approach nearer to that position the current in Lt 




induces more and more e. m. f . in coil L. For some 
position of the two coils, and the actual position de- 
pends on the tube we are using, there will be enough 
effect from the plate circuit upon the grid circuit so 
that there will be continuous oscillations. ' 

We want to stop just short of this position. 
There will then be no continuous oscillations; but 
if any changes do take place in the plate current 
they will affect the grid. And these changes in the 
grid voltage will result in still larger changes in the 
plate current. 

Now suppose that there is coming into the detector 
circuit of Fig. 92 a radio signal with speech signifi- 



178 LETTERS OF A RADIO-ENGINEER 

cance. The current in the plate circuit varies ac- 
cordingly. So does the current in coil Lt which is in 
the plate circuit. But this current induces an e. m. f. 
in coil L and this adds to the e. m. f. of the incom- 
ing signal so as to make a greater variation in the 
plate current. This goes on as long as there is an 
incoming signal. Because the plate circuit is coupled 
to the grid circuit the result is a larger e. m. f. in 
the grid circuit than the incoming signal could set 
up all by itself. 

You see now why I said the tube talked to itself. 
It repeats to itself whatever it receives. It has a 
greater strength of signal to detect than if it didn't 
repeat. Of course, it detects also just as I told you 
in the preceding letter. 

In adjusting the coupling of the two coils of Fig. 
92 we stopped short of allowing the tube circuit to 
oscillate and to generate a high frequency. If we 
had gone on increasing the coupling we should have 
reached a position where steady oscillations would 
begin. Usually this is marked by a little click in the 
receiver. The reason is that when the tube oscillates 
the average current in the plate circuit is not the 
same as the steady current which ordinarily flows 
between filament and plate. There is a sudden 
change, therefore, in the average current in the 
plate circuit when the tube starts to oscillate. You 
remember that what affects the receiver is the aver- 
age current in the plate circuit. So the receiver 
diaphragm suddenly changes position as the tube 
starts to oscillate and a listener hears a little click. 



THE REGENERATIVE CIRCUIT 179 

The frequency of the alternating current which 
the tube produces depends upon the tuned circuit 
formed by L and (7. Suppose that this frequency is 
not the same as that to which the receiving antenna 
is tuned. What will happen? 

There will be impressed on the grid of the tube 
two alternating e. m. f.'s, one due to the tube's own 
oscillations and the other incoming from the distant 
transmitting station. The two e. m. f.'s are both ac- 
tive at once so that at each instant the e. m. f. of 
the grid is really the sum of these two e m. f.'s. 
Suppose at some instant both e. m. f's are acting 
to make the grid positive. A little later one of them 
will be trying to make the grid negative while 
the other is still trying to make it positive. And 
later still when the first e. m. f. is ready again 
to make the grid positive the second will be trying to 
make it negative. 

It's like two men walking along together but with 
different lengths of step. Even if they start together 
with their left feet they are soon so completely out 
of step that one is putting down his right foot while 
the other is putting down his left. A little later, 
but just for an instant, they are in step again. And 
so it goes. They are in step for a moment and then 
completely out of step. Suppose one of them makes 
ten steps in the time that the other makes nine. In 
that time they will be once in step and once com- 
pletely out of step. If one makes ten steps while the 
other does eight this will happen twice. 

The same thing happens in the audion detector 



180 LETTERS OF A RADIO-ENGINEER 

circuit when two e. m. f.'s which differ slightly in 
frequency are simultaneously impressed on the grid. 
If one e. m. f. passes through ten complete cycles 
while the other is making eight cycles, then during 
that time they will twice be exactly in step, that is, 
*4n phase" as we say. Twice in that time they will 
be exactly out of step, that is, exactly ^* opposite in 
phase." Twice in that time the two e. m. f.'s will 
aid each other in their effects on the grid and twice 
they will exactly oppose. Unless they are equal 
in amplitude there will still be a net e. m. f. even 
when they are exactly opposed. The result of all 
this is that the average current in the plate circuit 
of the detector will alternately increase and decrease 
twice during this time. 

The listener will then hear a note of a frequency 
equal to the difference between the frequencies of the 
two e. m. f.'s which are being simultaneously im- 
pressed on the grid of the detector. Suppose the in- 
coming signal has a frequency of 100,000 cycles a 
second but that the detector tube is oscillating in its 
own circuit at the rate of 99,000 cycles per second, 
then the listener Avill hear a note of 1000 cycles per 
second. One thousand times each second the two e. m. 
f.'s will be exactly in phase and one thousand times 
each second they will be exactly opposite in phase. 
The voltage applied to the grid will be a maximum 
one thousand times a second and alternately a mini- 
mum. We can think of it, then, as if there were im- 
pressed on the grid of the detector a high-frequency 
signal which varied in intensity one thousand times a 



THE REGENERATIVE CIRCUIT 181 

second. This we know will produce a corresponding 
variation in the current through the telephone re- 
ceiver and thus give rise to a musical note of about 
two octaves above middle C on the piano. 

This circuit of Fig. 92 will let us detect signals 
which are not varying in intensity. And conse- 
quently this is the method which we use to detect 
the telegraph signals which are sent out by such a 
^^ continuous wave transmitter*' as I showed you at 
the end of Letter 13. 

When the key of a C-W transmitter is depressed 
there is set up in the distant receiving-antenna an al- 
ternating current. This current doesn't vary in 
strength. It is there as long as the sender has his 
key down. Because, however, of the effect which 
I described above there will be an audible note from 
the telephone receiver if the detector tube is oscil- 
lating at a frequency within two or three thousand 
cycles of that of the transmitting station. 

This method of receiving continuous wave signals 
is called the ' ^ heterodyne ' ' method. The name comes 
from two Greek words, ''dyne" meaning ''force'' 
and the other part meaning "different." We re- 
ceive by combining two different electron-moving- 
forces, one produced by the distant sending-station 
and the other produced locally at the receiving sta- 
tion. Neither by itself will produce any sound, ex- 
cept a click when it starts. Both together produce 
a musical sound in the telephone receiver; and the 
frequency of that note is the difference of the two 
frequencies. 



182 LETTERS OF A RADIO-ENGINEER 

There are a number of words "used to describe 
this circuit with some of which you should be famil- 
iar. It is sometimes called a "feed-back" circuit 
because part of the output of the audion is fed back 
into its input side. More generally it is known 
as the ''regenerative circuit" because the tube 
keeps on generating an alternating current. The 
little coil which is used to feed back into the grid 
circuit some of the effects from the plate circuit is 
sometimes called a ''tickler" coil. 

It is not necessary to use a grid condenser in a 
feed-back circuit but it is perhaps the usual method 
of detecting where the regenerative circuit is used. 
The whole value of the regenerative circuit so far 
as receiving is concerned is in the high efficiency 
which it permits. One tube can do the work of two. 

We can get just as loud signals by using another 
tube instead of making one do all the work. In the 
regenerative circuit the tube is performing two jobs 
at once. It is detecting but it is also amplifying. ^ 
By "amplifying" we mean making an e. m. f. larger 
than it is without changing the shape of its picture, 
that is without changing its "wave form." 

To show just what we mean by amplifying we must 
look again at the audion and see how it acts. You 
know that a change in the grid potential makes a 
change in the plate current. Let us arrange an au- 
dion in a circuit which will tell us a little more of 
what happens. Fig. 93 shows the circuit. 

1 There is always some amplification taking place in an audion 
detector but the regenerative circuit amplifies over and over again 
until the signal is as large as the tube can detect. 



THE REGENERATIVE CIRCUIT 



183 



This circuit is the same as we used to find the 
audion characteristic except that there is a clip for 
varying the number of batteries in the plate circuit 
and a voltmeter for measuring their e. m. f. We 
start with the grid at zero potential and the usual 
number of batteries in the plate circuit. The 
voltmeter tells us the e. m. f. We read the am- 
meter in the plate circuit and note what that cur- 
rent is. Then we shift the slider in the grid circuit 
so as to give the grid a small potential. The current 
in the plate circuit changes. We can now move the 




clip on the B-batteries so as to bring the current in 
this circuit back to its original value. Of course, if. 
we make the grid positive we move the clip so as to 
use fewer cells of the B-battery. On the other hand 
if we make the grid negative we shall need more e. 
m. f. in the plate circuit. In either case we shall 
find that we need to make a very much larger change 
in the voltage of the plate circuit than we have made 
in the voltage of the grid circuit. 

Usually we perform the experiment a little differ- 
ently so as to get more accurate results. We read 
the voltmeter in the plate circuit and the ammeter in 
that circuit. Then we change the number of batteries 
which we are using in the plate circuit. That changes 



184 LETTERS OF A RADIO-ENGINEER 

the plate current. The next step isto shift the slider 
in the grid circuit until we have again the original 
value of current in the plate circuit. Suppose that 
the tube is ordinarily run with a plate voltage of 40 
volts and we start with that e. m. f . on the plate. Sup- 
pose that we now make it 50 volts and then vary the 
position of the slider in the grid circuit until the am- 
meter reads as it did at the start. Next we read 
the voltage impressed on the grid by reading the 
voltmeter in the grid circuit. Suppose it reads 2 
volts. What does that meant 




^crccTxs/^ 



riQ s^ 



iii[Ml(lili|i|i|iP^ 



It means that two volts in the grid circuit have the 
same effect on the plate current as ten volts in the 
plate circuit. If we apply a volt tp the grid circuit 
we get five times as large an effect in the plate cir- 
cuit as we would if the volt were applied there. We 
get a greater effect, the effect of more volts, by apply- 
ing our voltage to the grid. We say that the tube 
acts as an ''amplifier of voltage'' because we can 
get a larger effect than the number of volts which 
we apply would ordinarily entitle us to. 

Now let's take a simple case of the use of an 
audion as an amplifier. Suppose we have a receiv- 
ing circuit with which we find that the signals are 



THE REGENERATIVE CIRCUIT 185 

not easily understood because they are too weak.. 
Let this be the receiving circuit of Fig. 88 which I am 
reproducing here as part of Fig. 94. 

We have replaced the telephone receiver by a 
' ^ transformer. ' ' A transformer is two coils, or wind- 
ings, coupled together. An alternating current in 
one w^U give rise to an alternating current in the 
other. You are already familiar with the idea but 
this is our first use of the word. Usually we call 
the first coil, that is the one through which the al- 
ternating current flows, the ** primary'' and the sec- 
ond coil, in which a current is induced, the *^ secon- 
dary. ' ' 

The secondary of this transformer is connected to 
the grid circuit of another vacuum tube, to the plate 
circuit of. which is connected another transformer 
and the telephone receiver. The result is a detector 
and ^^one stage of amplification." 

The primary of the first transformer, so we shall 
suppose, has in it the same current as would 
have been in the telephone. This alternating 
current induces in the secondary an e. m. f. 
which has the same variations as this current; This 

e. m. f . acts on the grid of the second tube, that is on 
the amplifier. Because the audion amplifies, the e. m. 

f. acting on the telephone receiver is larger than it 
would have been without the use of this audioi^. 
And hence there is a greater response on the part of 
its diaphragm and a louder sound. 

In setting up such a circuit as this there are sev- 
eral things to watch. For some of these you will 



186 LETTERS OF A RADIO-ENGINEER 

have to rely on the dealer from whom you buy your 
supplies and for the others upon yourself. But it 
will take another letter to tell you of the proper 
precautions in using an audion as an amplifier. 

In the circuit which I have just described an au- 
dion is used to amplify the current which comes from 
the detector before it reaches the telephone receiver. 
Sometimes we use an audion to amplify the e. m. f. 




/?^0/0 ^^Mf^L/F'teZf^ 



PETCCTOF^ 



of the signal before impressing it upon the grid of 
the detector. Fig. 95 shows a circuit for doing that. 
In the case of Fig. 94 we are amplifying the audio- 
frequency current. In that of Fig. 95 it is the radio- 
frequency effect which is amplified. The feed-back 
or regenerative circuit of Fig. 92 is a one-tube circuit 
for doing the same thing as is done with two tubes 
in Fig 95. 



LETTEE 1^ 

THE AUDION AMPLIFIER AND ITS 
CONNECTIONS 

Deae Son: 

In our use of the audion we form three circuits. 
The first or A-circuit includes the filament. The 
B-circuit includes the part of the tube between fila- 
ment and plate. The C-circuit includes the part be- 
tween filament and grid. We sometimes speak of the 
C-circuit as the '4nput" circuit and the B-circuit 
as the '' output'' circuit of the tube. This is because 
we can put into the grid-filament terminals an e. 
m. f. and obtain from the plate-filament circuit an 
effect in the form of a change of current. 

Suppose we had concealed in a box the audion and 
circuit of Fig. 96 and that only the terminals which 
are shown came through the box. We are given a 
battery and an ammeter and asked to find out all we 
can as to what is between the ter- 
minals F and G, We connect the 
battery and ammeter in series 
with these terminals. No current 
flows through the circuit. We re- 
verse the battery but no current 
flows in the opposite direction. Then we reason 
that there is an open-circuit between F and G. 

As long as we do not use a higher voltage than 

187 



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°i'':: 



188 LETTERS OF A RADIO-ENGINEER 

that of the C-battery which is in the box no current 
can flow. Even if we do use a higher voltage than 
the ''negative C-battery" of the hidden grid-circuit 
there will be a current only when the external bat- 
tery is connected so as to make the grid positive 
with respect to the filament. 

Now suppose we take several cells of battery and 
try in the same way to find what is hidden be- 
tween the terminals P and F. We start with one bat- 
tery and the ammeter as before and find that if this 
battery is connected so as to make P positive with 
respect to F, there is a feeble current. We increase 
the battery and find that the current is increased. 
Two cells, however, do not give exactly twice the cur- 
rent that one cell does, nor do three give three times 
as much. The current does not increase propor- 
tionately to the applied voltage. Therefore we rea- 
son that whatever is between P and F acts like a re- 
sistance but not like a wire resistance. 

Then, we try another experiment with this hidden 
audion. We connect a battery to G and F, and note 
what effect it has on the current which our other 
battery is sending through the box between P and F. 
There is a change of current in this circuit, just as 
if our act of connecting a battery to G-F had re- 
sulted in connecting a battery in series with the P-F 
circuit. The effect is exactly as if there is inside the 
box a battery which is connected into the hidden 
part of the circuit P-F. This concealed battery, 
which now starts to act, appears to be several times 
stronger than the battery which is connected to G-F, 



THE AUDION AMPLIFIER 189 

Sometimes this hidden battery helps the B-battery 
which is on the outside ; and sometimes it seems to 
oppose, for the current in the P-F circuit either in- 
creases or decreases, depending upon how we con- 
nect the battery to G and F. The hidden battery 
is always larger than our battery connected to G 
and F, If we arrange rapidly to reverse the bat- 
tery connected to G-F it appears as if there is in- 
side the box in the P-F circuit an alternator, that is, 
something which can produce an alternating e. m. f. 

All this, of course, is merely a review statement 
of what we already know. These experiments are in- 
teresting, however, because they follow somewhat 
those which were performed in studying the audion 
and finding out how to make it do all the wonderful 
things which it now can. 

As far as we have carried our series of experi- 
ments the box might contain two separate circuits. 
One between G and F appears to be an open circuit. 
The other appears to have in it a resistance and a 
battery (or else an alternator). The e. m. f. of the 
battery, or alternator, as the case may be, depends 
on what source of e. m. f . is connected to G-F. What- 
ever that e. m. f . is, there is a 
corresponding kind of e. m. f . 
inside the box but one several 
times larger. ^ 



c//?cu/rs o/^ 




We might, therefore, pay a%%SfF^^W^VS'L% 
no further attention to what ^^Q -^7 

is actually inside the box or how all these effects 
are brought about. We might treat the entire box 



190 LETTERS OF A RADIO-ENGINEER 

as if it was formed by two separate circuits as shown 
in Fig. 97. If we do so, we are replacing the box 
by something which is equivalent so far as effects 
are concerned, that is we are replacing an actual au- 
dion by two circuits which together are equivalent to 
it. 

The men who first performed such experiments 
wanted some convenient way of saying that if an 
alternator, which has an e. m. f. of V volts, is con- 
nected to F and G, the effect is the same as if a much 
stronger alternator is connected between F and P. 
How much stronger this imaginary alternator is de- 
pends upon the design of the audion. For some au- 
dions it might be five times as strong, for other 
designs 6.5 or almost any other number, although 
usually a number of times less than 40. They used a 
little Greek letter called ^'mu^' to stand for this 
number which depends on the design of the tube. 
Then they said that the hidden alternator in the 
output circuit was mu times as strong as the actual 
alternator which was applied between the grid and 
the filament. Of course, instead of writing the sound 
and name of the letter they used the letter M itself. 
And that is what I have done in the sketch of Fig. 97. 

Now we are ready to talk about the audion as an 
amplifier. The first thing to notice is the fact that we 
have an open circuit between F and G. This is true 
as long as we don't apply an e. m. f. large enough 
to overcome the C-battery of Fig. 96 and thus let the 
grid become positive and attract electrons from the 
filament. We need then spend no further time think- 



THE AUDION AMPLIFIER 191 

ing about what will happen in the circuit G-F^ for 
there will be no current. 

As to the circuit F-P, we can treat it as a resist- 
ance in series with which there is a generator /* 
times as strong as that which is connected to F and G. 
The next problem is how to get the most out of this 
hidden generator. We call the resistance which the 
tube offers to the passage of electrons between P and 
F the 'internal resistance" of the plate circuit of 
the tube. How large it is depends upon the design 
of tube. In some tubes it may be five or six thousand 
ohms, and in others several times as high. In the 
large tubes used in high-powered transmitting sets 
it is much less. Since it will be different in different 
cases we shall use a symbol for it and say that it is 
Rv ohms. 

Then one rule for using an audion as an amplifier 
is this : To get the most out of an audion see that 
the telephone, or whatever circuit or piece of ap- 
paratus is connected to the output terminals, shall 
have a resistance of Rp ohms. When the resistance 
of the circuit, which an audion is supplying with cur- 
rent, is the same as the internal resistance of the 
output side of the tube, then the audion gives its 
greatest output. That is the condition for the great- 
est '^amount of energy each second," or the ^^ great- 
est power" as we say. 

That rule is why we always select the telephone 
receivers which we use with an audion and always 
ask carefully as to their resistance when we buy. 
Sometimes, however, it is not practicable to use re- 



192 LETTERS OF A RADIO-ENGINEER 

ceivers of just the right resistance. Where we con- 
nect the output side of an audion to some other cir- 
cuit, as where we let one audion supply another, it 
is usually impossible to follow this rule without add- 
ing some special apparatus. 

This leads to the next rule: If the telephone re- 
ceiver, or the circuit, which we wish to connect to 
the output of an audion, does not have quite nearly 
a resistance of Rp ohms we use a properly designed 
transformer as we have already done in Figs. 94 
and 95. 

A transformer is two separate coils coupled to- 
gether so that an alternating current in the primary 
will induce an alternating current in the secondary. 
Of course, if the secondary is open-circuited then 
no current can flow but there will be induced in it an 
e. m. f . which is ready to act if the circuit is closed. 
Transformers have an interesting ability to make 
a large resistance look small or vice versa. To 
show you why, I shall have to develop some rules for 
transformers. 

Suppose you have an alternating e. m. f. of ten 
volts applied to the primary of an iron-cored trans- 
former which has ten turns. There is one volt ap- 
plied to each turn. Now, suppose the secondary has 
only one turn. That one turn has induced in it an 
alternating e. m. f. of one volt. If there are more 
turns of wire forming the secondary, then each turn 
has induced in it just one volt. But the e. m. f.'s of 
all these turns add together. If the secondary has 
twenty turns, there is induced in it a total of twenty 



THE AUDION AMPLIFIER 193 

volts. So the first rule is this: In a transformer 
the number of volts in each turn of wire is just the 
same in the secondary as in the primary. 

If we want a high-voltage alternating e. m. f. 
all we have to do is to send an alternating current 
through the primary of a transformer which has in 
the secondary, many times more turns of wire than 
it has in the primary. From the secondary we ob- 
tain a higher voltage, than we impress on the pri- 
mary. 

You can see one application of this rule at once. 
When we use an audion as an amplifier of an alter- 
nating current we send the current which is to be am- 
plified through the primary of a transformer, as in 
Fig. 94. We use a transformer with many times 
more turns on the secondary than on the primary 
so as to apply a large e. m. f. to the grid of the 
amplifying tube. That will mean a large effect in 
the plate circuit of the amplifier. 

You remember that the grid circuit of an audion 
with a proper value of negative C-battery is really 
open-circuited and no current will flow in it. For 
that case we get a real gain by using a ^^ step-up'' 
transformer, that is, one with more turns in the sec- 
ondary than in the primary. 

It looks at first as if a transformer would al- 
ways give a gain. If we mean a gain m energy it 
will not although we may use it, as we shall see in a 
minute, to permit a vacuum tube to work into an 
output circuit more efficiently than it could without 
the transformer. We cannot have any more energy 



194 LETTERS OF A RADIO-ENGINEER 

in the secondary circuit of a transformer than we 
give to the primary. 

Suppose we have a transformer with twice as 
many turns on the secondary as on the primary. To 
the primary we apply an alternating e. m. f. of a 
certain number of volts. In the secondary there 
will be twice as many volts because it has twice as 
many turns. The current in the secondary, however, 
will be only half as large as is the current in the 
primary. We have twice the force in the secondary 
but only half the electron stream. 

It is something like this : You are out coasting 
and two youngsters ask you to pull them and their 
sleds up hill. You pull one of them all the way and 
do a certain amount of work. On the other hand 
suppose you pull them both at once but only half way 
up. You pull twice as hard but only half as far and 
you do the same amount of work as before. 

We can't get more work out of the secondary of 
a transformer than we do in the primary. If we 
design the transformer so that there is a greater 
Tuffrf^ pull (e. m. f.) in the secondary 

the electron stream in the sec- 
ondary will be correspondingly 
smaller. 
'21 ^^^ remember how we meas- 
s.:t T^m^rof^MER ^^^ resistance. We divide the 
^xi -- ^erx ai e. m. f . (number of volts) by 

''^ -^^ the current (number of am- 

peres) to find the resistance (number of ohms). 
Suppose we do that for the primary and for the 




'/^€ 



?^ 



THE AUDION AMPLIFIER 



195 



T-uffnsfQ 




secondary of the transformer of Fig. 98 which we 

are discussing. See what happens in the secondary. 

There is only half as much voltage but twice as much 

current. It looks as though the 

secondary had one-fourth as 

much resistance as the primary. 

And so it has, hut we usually 

call it 'impedance'' instead of 

resistance because straight wires 

resist but coils or condensers ^ex-r*- ^^ ^ ^/ 

impede alternating e. m. f.'s. /Q :y:y 

Before we return to the question of using a trans- 
former in an audion circuit let us turn this trans- 
former around as in Fig. 99 and send the current 
through the side with the larger number of windings. 
Let's talk of ^'primary" and ^^ secondary'' just as 
before but, of course, remember that now the pri- 
mary has twice the turns of the secondary. On the 
secondary side we shall have only half the current, 
but there will be twice the e. m. f. The resistance 
of the secondary then is four times that of the 
primary. 

Now return to the amplifier of Fig. 94 and see what 
sort of a transformer should be between the plate 
circuit of the tube and the telephone receivers. Sup- 
pose the internal resistance of the tube is 12,000 ohms 
and the resistance of the telephones is 3,000 ohms. 
Suppose also that the resistance (really impedance) 
of the primary side of the transformer which we 
just considered is 12,000 ohms. The impedance of 
its secondary will be a quarter of this or 3,000 ohms. 



i 



196 LETTERS OF A RADIO-ENGINEER 

If we connect such a transformer in the circuit, as 
shown, we shall obtain the greatest output from the 
tube. 

In the first place the primary of the transformer" 
has a number of ohms just equal to the internal re- 
sistance of the tube. The tube, therefore, will give 
its best to that transformer. In the second place 
the secondary of the transformer has a resistance 
just equal to the telephone receivers so it can give 
its best to them. The effect of the transformer is to 
make the telephones act as if they had four times as 
much resistance and so were exactly suited to be 
connected to the audion. 

This whole matter of the proper use of trans- 
formers is quite simple but very important in setting 
up vacuum-tube circuits. To overlook it in building 
or buying your radio set will mean poor efficiency. 
Whenever you have two parts of a vacuum-tube cir- 
cuit to connect together be sure and buy only a trans- 
former which is designed to work between the two 
impedances (or resistances) which you wish to con- 
nect together. 

There is one more precaution in connection with 
the purchase of transformers. They should do the 
same thing for all the important frequencies which 
they are to transmit. If they do not, the speech or 
signals will be distorted and may be unintelligible. 

If you take the precautions which I have mentioned 
your radio receiving set formed by a detector and one 
amplifier will look like that of Fig. 94. That is only 
one possible scheme of connections. You can use 



THE AUDION AMPLIFIER 



197 



any detector circuit which you wish, ^ one with a 
grid condenser and leak, or one arranged for feed- 




back. In either case your amplifier may well be as 
shown in the figure. 

1 Except for patented circuits. See p. 224. 



198 LETTERS OF A RADIO-ENGINEER 

The circuit I have described uses an audion to 
amplify the audio-frequency currents which come 
from the detector and are capable of operating the 
telephones. In some cases it is desirable to amplify 
the radio signals before applying them to the detec- 
tor. This is especially true where a '4oop antenna" 
is being used. Loop antennas are smaller and more 
canvenient than aerials and they also have certain 
abilities to select the sigTials which they are to re- 
ceive because they receive best from stations which 
lie along a line drawn parallel to their turns. Unfor- 
tunately, however, they are much less efficient and 
so require the use of amplifiers. 

With a small loop made by ten turns of wire sep- 
arated by about a quarter of an inch and wound on 
a square mounting, about three feet on a side, you will 
usually require two amplifiers. One of these might 
be used to amplify the radio signals before detection 
and the other to amplify after detection. To tune the 
loop for broadcasts a condenser of about 0.0005 mf. 
will be needed. The diagram of Fig. 100 shows the 
complete circuit of a set with three stages of radio- 
amplification and none of audio. 



LETTER 20 



TELEPHONE RECEIVERS AND OTHER 
ELECTROMAGNETIC DEVICES 

Deae Son: 

In an earlier letter when we first introduced a 
telephone receiver into a circuit I told you something 
of how it operates. I want now to tell why and also 
of some other important devices which operate for 
the same reason. 

You remember that a stream 
of electrons which is starting or 
stopping can induce the electrons 
of a neighboring parallel circuit 
to start off in parallel paths. 
We do not know the explanation 
of this. Nor do we know the ex- 
planation of another fact which 
seems to be related to this fact 
of induction and is the basis for 
our explanations of magnetism. 

If two parallel wires are carry- 
ing steady electron streams in the 
same general direction the wires 
attract each other. If the streams are oppositely 
directed the wires repel each other. Fig. 101 illus- 

199 




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f^/Q /o/ 



200 LETTERS OF A RADIO-ENGINEER 




/ b 



I! 



fl ^ 



^ y 



trates this fact. If the streams are not at all in the 
same direction, that is, if they are at right angles, 
they have no effect on each other. 

These facts, of the attrac- 
tion of electron streams 
which are in the same direc- 
tion and repulsion of streams 
in opposite directions, are 
all that one need remember 
to figure out for himself what 
will happen under various 
conditions. For example, if 
two coils of wire are carry- 
ing currents what will happen 
is easily seen. Fig. 102 
shows the two coils and a 
section through them. 

Looking at this cross sec- 
tion we seem to have four 
wires, 1 and 2 of coil A and 3 and 4 of coil B. You 
see at once that if the coils are free to move they 
will move into the dotted posi- 
tions shown in Fig 102, because 
wire ^ attracts wire 3 and re- 
pels wire 4y while wire 2 at- 
tracts wire 4 and repels wire 3. 
If necessary, and if they are 
free to move, the coils will turn 'J /^f'^^^^ 
completely around to get to this ^tt^rct^S/^ 
position. I have shown such a /^/Q /03 
case in Fig. 103. 



/7 R' 3' B 
/^rT/V/^c -r/ory 



F'/Q /oa 




ELECTROMAGNETIC DEVICES 201 

Wires which are not carrying currents do not 
behave in this way. The action is due, but how we 
don't yet know, to the motions of the electrons. As 
far as we can explain it to-day, the attraction of two 
wires which are carrying currents is due to the at- 
traction of the two streams of electrons. Of course 
these electrons are part of the wires. They can't get 
far away from the stay-at-home electrons and the 
nuclei of the atoms which form the wires. In fact 
it is these nuclei which keep the wandering electrons 
within the wires. The result is that if the streams 
of electrons are to move toward each other the wires 
must go along with them. 

If the wires are held firmly the electron streams 
cannot approach one another for they must stay in 
the wires. Wires, therefore, perform the impor- 
tant service of acting as paths for electrons which 
are traveling as electric currents. There are other 
ways in which electrons can be kept in a path, and 
other means beside batteries for keeping them going. 
It doesn't make any difference so far as the attrac- 
tion or the repulsion is concerned why they are fol- 
lowing a certain path or why they stay in it. So far 
as we know two streams of electrons, . ^ „ 
following parallel paths, will always ^^ W» 
behave just like the two streams of ^^ '^ ^ { *, ' 
Fig. 101. ^^/ ^y 

Suppose, for example, there were 
two atoms which were each formed /^/<? /O^ 
by a nucleus and a number of electrons swing- 
ing around about the nucleus as pictured in 



^02 LETTERS OF A RADIO-ENGINEER 

Fig. 104. The electrons are going of their own 
accord and the nucleus keeps them from flying off at 
^ ^ ./"S >% ^ tangent, the way mud flies 
^{ it ^ I \ fi'om the wheel of an auto- 
^^^y a i^ ^^ i^ mobile. Suppose these two 
^•/ ^4/ v/ ^J atoms are free to turn but 
not to move far from their 
° ^ ^ ° present positions. They will 

» ♦ • • turn so as to make their 

electron paths parallel just 
Vc/.^cr-;.^ a« did the loops of Fig. 102. 
/^ro/^^{^^^/=fz.L Now, I don't say that there 

/=ys /^ r^i^tQ/yer are any atoms at all like the 

f^/Q /0>5 ones I have pictured. There 

is still a great deal to be learned about how elec- 
trons act inside different kinds of atoms. We do 
know, however, that the atoms of iron act just as if 
they were tiny loops with electron streams. 

Suppose we had several loops and that they were 
lined up like the three loops in Fig. 105. You can 
see that they would all attract the other loop, on the 
right in the figure. On the other hand if they 
were grouped in the triangle of 
Fig. 106 they would barely affect 'i ^^:^ [[ 

the loop because they would be h ^o H 

pulling at cross purposes. If a J ^^ I. 

lot of the tiny loops of the iron ^rons //v ^/y 
atoms are Imed up so as to act /^/^/^/^ntQerr^er/yr 
together and attract other loops, /^/ Q /OO 
as in the first figure, we say the iron is magnetized 
and is a magnet. In an ordinary piece of iron, 




Pl. IX. Western Electric Loud Speaking Receiver. 

Crystal Detector Set of the General Electric Co. 
Audibility Meter of General Kadio Co. 



ELECTROMAGNETIC DEVICES 



203 



M 



5 n 



f^TTRRCTIOri 

q 



however, the atoms are so grouped that they don't 
pull together but like the loops of our second figure 
pull in different directions and neutralize each 
other's efforts so that there is no net effect. 

And like the loops of Fig. 106 the atoms in an 
unmagnetized piece of iron are pretty well satisfied 
to stay as they are without all lining up to pull to- 
gether. To magnetize the iron we must force some of 
these atomic loops to turn part way around. That 
can be done by bringing near 
them a strong magnet or a 
coil of wire which is carrying 
a current. Then the atoms are 
forced to turn and if enough 
turn so that there is an appre- 
ciable effect then the iron is 
magnetized. The more that are 
properly turned the stronger 
is the magnet. One end or 
**pole" we call north-seeking 
and the other south- seeking, 
because a magnetized bar of iron acts like a com- 
pass needle. 

A coil of wire, carrying a current, acts just like 
a magnet because its larger loops are all ready to 
pull together. I have marked the coil of Fig. 107 
with iV and B for north and south. If the electron 
stream in it is reversed the ^* polarity" is reversed. 
There is a simple rule for this. Partially close your 
left hand so that the fingers form loops. Let the 
thumb stick out at right angles to these loops. If the 



N ri 



C/70SS SCCTION OF- 



204 LETTERS OF A RADIO-ENGINEER 

electron streams are flowing around the loops of a 
coil in the same direction as your fingers point then 
your thumb is the N pole and the coil will repel the 
north poles of other loops or magnets in the direction 
in which your thumb points. If you know the pol- 
arity already there is a simple rule for the repulsion 
or attraction. Like poles repel, unlike poles attract. 

From what has been said about magnetism you can 
now understand why in a telephone receiver the cur- 
rent in the winding can make the magnet stronger. 
It does so because it makes more of the atomic loops 
of the iron turn around and help pull. On the other 
hand if the current in the winding is reversed it 
will turn some of the loops which are already help- 
ing into other positions where they don't help and 
may hinder. If the current in the coil is to help, the 
electron stream in it must be so directed that the 
north pole of the coil is at the same end as the north 
pole of the magnet. 

This idea of the attraction or repulsion of electron 
streams, whether in coils of wire or in atoms of iron 
and other magnetizable substances, is the funda- 
mental idea of most forms of telephone receivers, of 
electric motors, and of a lot of other devices which 
we call ** electromagnetic. " 

The ammeters and voltmeters which we use for 
the measurement of audion characteristics and the 
like are usually electromagnetic instruments. Am- 
meters and voltmeters are alike in their design. 
Both are sensitive current-measuring instruments. 
In the case of the voltmeter, as you know, we have 



ELECTROMAGNETIC DEVICES 205 

a large resistance in series with the current-measur- 
ing part for the reason of which I told in Letter 8. 
In the case of ammeters we sometimes let all the 
current go through the current-measuring part but 
generally we let only a certain fraction of it do so. 
To pass the rest of the current we connect a small 
resistance in parallel with the measuring part. In 
both types of instruments the resistances are some- 
times hidden away under the cover. Both instru- 
ments must, of course, be calibrated as I have ex- 
plained before. 

In the electromagnetic instruments there are sev- 
eral ways of making the current-measuring part. 
The simplest is to let the current, or part of it, flow 
through a coil which is pivoted between the N and S 
poles of a strong permanent magnet. A spring 
keeps the coil in its zero position and if the current 
makes the coil turn it must do so against this spring. 
The stronger the current in the coil the greater 
the interaction of the loops of the coil and those of 
the iron atoms and hence the further the coil will 
turn. A pointer attached to the coil indicates how 
far ; and the number of volts or amperes is read off 
from the calibrated scale. 

Such instruments measure direct-currents, that is, 
steady streams of electrons in one direction. To 
measure an alternating current or voltage we can 
use a hot-wire instrument or one of several different 
types of electromagnetic instruments. Perhaps the 
simplest of these is the so-called ^^ plunger type.'' 
The alternating current flows in a coil ; and a piece of 



206 LETTERS OF A RADIO-ENGINEER 

soft iron is so pivoted that it can be attracted and 
moved into the coil. Soft iron does not make a 
good permanent magnet. If you put a piece of it 
inside a coil which is carrying a steady current it 
becomes a magnet but about as soon as you interrupt 
the current the atomic loops of the iron stop pulling 
together. Almost immediately they turn into all 
sorts of positions and form little self-satisfied groups 
which don't take any interest in the outside world. 
(That isn't true of steel, where the atomic loops 
are harder to turn and to line up, but are much more 
likely to stay in their new positions.) 

Because the plunger in an alternating-current am- 
meter is soft iron its loops line up with those of the 
coil no matter which way the electron stream happens 
to be going in the coil. The atomic magnets in the 
iron turn around each time the current reverses and 
they are always, therefore, lined up so that the 
plunger is attracted. If the plunger has much inertia 
or if the oscillations of the current are reasonably 
frequent the plunger will not move back and forth 
with each reversal of the current but will take an 
average position. The stronger the a-c (alternating 
current) the farther inside the coil will be this posi- 
tion of the plunger. The position of the plunger 
becomes then a measure of the strength of the alter- 
nating current. 

Instruments for measuring alternating e. m. f.'s 
and currents, read in volts and in amperes. So far I 
haven't stopped to tell what we mean by one ampere 
of alternating current. You know from Letter 7 



ELECTROMAGNETIC DEVICES 207 

what we mean by an ampere of d-c (direct current). 
It wasn't necessary to explain before because I told 
you only of hot-wire instruments and they will read 
the same for either d-c or a-c. 

When there is an alternating current in a wire the 
electrons start, rush ahead, stop, rush back, stop, and 
do it all over again and again. That heats the wire 
in which it happens. If an alternating stream of 
electrons, which are doing this sort of thing, heats a 
wire just exactly as much as would a d-c of one 
ampere, then we say that the a-c has an *^ effective 
value'' of one ampere. Of course part of the time 
of each cycle the stream is larger than an ampere 
but for part it is less. If the average heating effect 
is the same the a-c is said to be one ampere. 

In the same way, if a steady e. m. f. (a d-c e. m. f.) 
of one volt will heat a wire to which it is applied a 
certain amount and if an alternating e. m. f. will have 
the same heating effect in the same time, then the a-c 
e. m. f. is said to be one volt. 

Another electromagnetic instrument which we 
have discussed but of which more should be said is 
the iron-cored transformer. We consider first what 
happens in one of the coils of the transformer. 

The inductance of a coil is very much higher if it 
has an iron core. The reason is that then the coil 
acts as if it had an enormously larger number of 
turns. All the atomic loops of the core add their 
effects to the loops of the coil. When the current 
starts it must line up a lot of these atomic loops. 
When the current stops and these loops turn back 



208 LETTERS OF A RADIO-ENGINEER 



^ 




into some of their old self-satisfied groupings, they 

affect the electrons in the coil. 

Where first they opposed the 

motion of these electrons, now 

they insist on its being continued 

for a moment longer. I'll prove 

that by describing two simple ex- 

r/Q 3^ periments; and then we'll have 

the basis for understanding the effect of an iron 

core in a transformer. 

Look again at Fig. 33 of Letter 9 which I am re- 
producing for convenience. We considered only what 
would happen in coil cd if a current was started in 
coil ah. Suppose instead of placing the coils as 
shown in that figure they are placed as in Fig. 108. 
Because they are at right angles there will be no 
effect in cd when the current is started in ab. Let 
the current flow steadily through db and then sud- 
denly turn the coils so that they are again parallel 
as shown by the dotted positions. We get the same 
temporary current in cd as we would if we should 
place the coils parallel and ^^ TL 

then start the current in ah, 1= (/) 

The other experiment is c IS T 

this: Starting with the coils 
lined up as in the dotted po- 
sition of Fig. 108 and the 
current steadily flowing in 
ah, we suddenly turn them F'/Q /0& 

into positions at right angles to each other. There 
is the same momentary current in cd as if we had 




ELECTROMAGNETIC DEVICES 209 

left them lined up and had opened the switch in 
the circuit of ah. 

Now we know that the atomic loops of iron behave 
in the same general way as do loops of wire which 
are carrying currents. Let us re- 
place the coil ah by a magnet as 
shown m Fig. 109. First we start 
with the magnet at right angles to 
the coil cd. Suddenly we turn it 
into the dotted position of that 
... -- /^^ figure. There is the same momen- 
tary current m cd as if we were 
still using the coil ab instead of a magnet. If now 
we turn the magnet back to a position at right 
angles to cd, we observe the opposite direction of 
current in cd. These effects are more noticeable 
the more rapidly we turn the magnet. The same is 
true of turning the coil. 

The experiment of turning the magnet illustrates 
just what happens in the case of a transformer with 
an iron core except that instead of turning the entire 
magnet the little atomic loops do the turning inside 
the core. In the secondary of an iron-cored trans- 
former the induced current is the sum of two cur- 
rents both in the same direction at each instant. 
One current is caused by the starting or stopping 
of the current in the primary. The other current is 
due to the turning of the atomic loops of the iron 
atoms so that more of them line up with the turns of 
the primary. These atomic loops, of course, are 
turned by the current in the primary. There are so 



210 LETTERS OF A RABIO-ENGINEER 

many of them, however, that the current due to their 
turning is usually the more important part of the 
total current. 

In all transformers the effect is greater the more 
rapidly the current changes direction and the atomic 
loops turn around. For the same size of electron 
stream in the primary, therefore, there is induced 
in the secondary a greater e. m. f . the greater is the 
frequency with which the primary current alter- 
nates. 

Where high frequencies are dealt with it isn't 
necessary to have iron cores because the effect is 
large enough without the help of the atomic loops. 
And even if we wanted their help it wouldn't be 
easy to obtain, for they dislike to turn so fast and it 
takes a lot of power to make them do so. We know 
that fact because we know that an iron core in- 
creases the inductance and so chokes the current. 
For low frequencies, however, that is those fre- 
quencies in the audio range, it is usually necessary 
to have iron cores so as to get enough effect without 
too many turns of wire. 

The fact that iron decreases the inductance and 
so seriously impedes alternating currents leads us 
to use iron-core coils where we want high inductance. 
Such coils are usually called ^' choke coils" or ^'re- 
tard coils. ' ' Of their use we shall see more in a later 
letter where we study radio-telephone transmitters. 



LETTER 21 

YOUR RECEIVING SET AND HOW TO 
EXPERIMENT 

My Deak Student : 

In this letter I want to tell you how to experi- 
ment with radio apparatus. The first rule is this : 
Start with a simple circuit, never add anything to 
it until you know just why you are doing so, and 
do not box it up in a cabinet until you know how it is 
working and why. 

Your antenna at the start had better be a single 
wire about 25 feet high and about 75 feet long. 
This antenna will have capacity of about 0.0001 m. f. 
If you want an antenna of two wires spaced about 
three feet apart I would make it about 75 feet long. 
Bring down a lead from each wire, twisting them 
into a pigtail to act like one wire except near the hori- 
zontal part of the antenna. 

Your ground connection can 

go to a water pipe. To protect 

the house and your apparatus 

from lightning insert a tuse and 

J^Ga7fS^^^lSf/;y§ a little carbon block lightning 

/vd? //O arrester such as are used by 

the telephone company in their 

installations of house phones. You can also use a 

so-called ** vacuum lightning arrester.'' In either 

211 




212 



LETTERS OF A RADIO-ENGINEER 



case the connections will be as shown in Fig. 111. 
If you use a loop antenna, of course, no arrester is 
needed. 

At first I would plan to receive signals between 150 
meters and 360 meters. This will include the ama- 
teurs who work between 160 and 200 m., the special 
amateurs who send C-W telegraph at 275 m., and the 
broadcasting stations which operate at 360 m. This 
range will give you plenty to listen to while you are 




/P£:cs/y/r/0 



F'/q /// 



experimenting. In addition you will get some ship 
signals at 300 m. 

To tune the antenna to any of the wave lengths in 
this range you can use a coil of 75 turns wound on 
a cardboard tube of three and a half inches in diam- 
eter. You can wind this coil of bare wire if you 
are careful, winding a thread along with the wire 
so as to keep the successive turns separated. In that 
case you will need to construct a sliding contact for 
it. That is the simplest form of tuner. 

On the other hand you can wind with single silk 
covered wire and bring out taps at the 0, 2, 4, 6, 8, 



HOW TO EXPERIMENT 213 

10, 14, 20, 28, 36, 44, 56, 66, and 75tli turns. To make 
a tap drill a small hole through the tube, bend the 
wire into a loop about a foot long and pull this loop 
through the hole as shown in Fig. 110. Then give 
the wire a twist, as shown, so that it can't pull out, 
and proceed with your winding. 

Use 26 s. s. c. wire. You will need about 80 feet 
and might buy 200 to have enough for the secondary 
coil. Make contacts to the taps by two rotary 
switches as shown in Fig. 112. You can buy switch 
arms and contacts studs or a complete switch 
mounted on a small panel of some insulating com- 
pound. Let switch Si make the contacts for taps 
between 14 and 75 turns, and let switch 52 make the 
other contacts. 

For the secondary coil use the same size of wire 
and of core. Wind 60 turns, bringing out a tap 
at the middle. To tune the secondary circuit you 
will need a variable condenser. You can buy one of 
the small ones with a maximum capacity of about 
0.0003 mf., one of the larger ones with a maximum 
capacity of 0.0005 mf., or even the larger size which 
has a maximum capacity of 0.001 mf . I should prefer 
the one of 0.0005 mf. 

You will need a crystal detector — I should try 
galena first — and a so-called ''cat's whisker" with 
which to make contact with the galena. For these 
parts and for the switch mentioned above you can 
shop around to advantage. For telephone receivers 
I would buy a really good pair with a resistance of 
about 2500 ohms. Buy also a small mica condenser 



214 LETTERS OF A RADIO-ENGINEER 



of 0.002 mf. for a blocking condenser. Your entire 
outfit will then look as in Fig. 112. The switch S is 
a small knife switch. 

To operate, leave the switch S open, place the 
primary and secondary coils near together as in the 
figure and listen. The tuning is varied, while you 
listen, by moving the slider of the slide-wire tuner 

or by moving the switches 
if you have connected your 
coil for that method. Make 
large changes in the tuning 
by varying the switch 5i and 
then turn slowly through all 
positions of ^2, listening at 
each position. 

When a signal is heard 
/v^ //2 adjust to the position of 5i 

and ^2 which gives the loudest signal and then clos- 
ing S start to tune the secondary circuit. To do 
this, vary the capacity of the condenser in the secon- 
dary circuit. Don't change the primary tuning until 
you have tuned the secondary and can get the signal 
with good volume, that is loud. You will want to 
vary the position of the primary and secondary coils, 
that is, vary their coupling, for you will get sharper 
tuning as they are drawn farther apart. Sharper 
tuning means less interference from other stations 
which are sending on wave lengths near that which 
you wish to receive. Reduce the coupling, therefore, 
and then readjust the tuning. It will usually be neces- 
sary to make a slight change in both circuits, in one 




HOW TO EXPERIMENT 215 

case with switch Si and in the other with the variable 
condenser. 

As soon as yon can identify any station which yon 
hear sending make a note of the position of the 
switches Si and 52, and of the setting of the condenser 
in the secondary circuit. In that way yon will ac- 
quire information as to the proper adjustments to 
receive certain wave-lengths. This is calibrating 
your set by the known wave-lengths of distant sta- ^ 

tions. 

After learning to receive with this simple set 
I should recommend buying a good audion tube. Ask 
the seller to supply you with a blue print of the 
characteristic ^ of the tube taken under the condi- 
tions of filament current and plate voltage which he 
recommends for its use. Buy a storage battery and 
a small slide-wire rheostat, that is variable resist- 
ance, to use in the filament circuit. Buy also a bank 
of dry batteries of the proper voltage for the plate 
circuit of the tube. At the same time you should buy 
the proper design of transformer to go between the 
plate circuit of your tube and the pair of receivers 
which you have. It will usually be advisable to ask 
the dealer to show you a characteristic curve for 
the transformer, which will indicate how well the 
transformer operates at the different frequencies in 
the audio range. It should operate very nearly the 
same for all frequencies between 200 and 2500 cycles. 

The next step is to learn to use the tube as a de- 

1 If you can afford to buy, or if you can borrow, ammeters and 
voltmeters of the proper range you should take the characteristic 
yourself. 



216 LETTERS OF A RADIO-ENGINEER 

tector. Connect it into yonr secondary circuit in- 
stead of the crystal detector. Use the proper value 
of C-battery as determined from your study of the 
characteristic of the tube. One or two small dry 
cells, which have binding-post terminals are con- 
venient C-batteries. If you think you will need a 
voltage much different from that obtained with a 
whole number of batteries you can arrange to supply 
the grid as we did in Fig. 86 of Letter 18. In that 
case you can use a few feet of 30 German-silver wire 
and make connections to it with a suspender clip. 
Learn to receive with the tube and be particularly 
careful not to let the filament have too much current 
and burn out. 

Now buy some more apparatus. You will need a 
grid condenser of about 0.0002 mf. The grid leaks 
to go with it you can make for yourself. I would use 
a piece of brown wrapping paper and two little 
metal eyelets. The eyelets can be punched into the 
paper. Between them coat the paper with carbon 
ink, or with lead pencil marks. A line about an inch 
long ought to serve nicely. You will probably wish 
to make several grid leaks to try. "When you get sat- 
isfactory operation in receiving by the grid-con- 
denser method the leak will probably be somewhere 
between a megohm and two megohms. 

For this method you will not want, a C-battery, 
but you will wish to operate the detector with about 
as high a voltage as the manufacturers will recom- 
mend for the plate circuit. In this way the incom- 
ing signal, which decreases the plate current, can 



HOW TO EXPERIMENT 



217 



produce the largest decrease. It is also possible to 
start with the grid slightly positive instead of being 
as negative as it is when connected to the negative 
terminal of the A-battery. There will then be pos- 
sible a greater change in grid voltage. To do so 
connect the grid as in Fig. 115 to the positive termi- 
nal of the A-battery. 

About this time I would shop around for two or 
three small double-pole double-throw switches. 

















n^ 1/3 



BILITY 



j 



Those of the 5-ampere size will do. With these you 
can arrange to make comparisons between different 
methods of receiving. Suppose, for example, you 
connect the switches as shown in Fig. 113 so that by 
throwing them to the left you are using the audion 
and to the right the crystal as a detector. You can 
listen for a minute in one position and then switch 
and listen for a minute in the other position, and 
so on back and forth. That way you can tell whether 
or not you really are getting better results. 

If you want a rough measure of how much better 
the audion is than the crystal you might see, while 
you are listening to the audion, how much you can 



218 LETTERS OF A RADIO-ENGINEER 

rob the telephone receiver of its current and still 
hear as well as you do when you switch back to the 
crystal. The easiest way to do this is to put a 
variable resistance across the receiver as shown in 
Fig. 113. Adjust this resistance until the intensity 
of the signal when detected by the audion is the 
same as for the crystal. You adjust this variable 
resistance until it by-passes so much of the current, 
which formerly went through the receiver, that the 
^^ audibility '^ of the signal is reduced until it is the 
same as for the crystal detector. Carefully made 
resistances for such a purpose are sold under the 
name of ^^ audibility meters. '^ You can assemble 
a resistance which will do fairly well if you will buy 
a small rheostat which will give a resistance vary- 
ing by steps of ten ohms from zero to one hundred 
ohms. At the same time you can buy four resistance 
spools of one hundred ohms each and perhaps one 
of 500 ohms. The spools need not be very expensive 
for you do not need carefully adjusted resistances. 
Assemble them so as to make a rheostat with a 
range of 0-1000 ohms by steps of 10 ohms. The 
cheapest way to mount is with Fahnestock clips as 
^r^,^r^nc^ illustrated in Fig. 114. ^ After a 

^/:>oocs ,(=3 =* while, however, you will prob- 
ably wish to mount them in a 
box with a rotary switch on top. 
(k^Htvc^oc^ cc/^s To study the effect of the 
/v<^ //^ grid condenser you can arrange 

switches so as to insert this condenser and its leak 
and at the same time to cut out the C-battery. Fig. 





|u 



Pl. X. — Audio-frequency Transforaier 
AND Banked-wound Coil. ( Courtesy of 
Pacent Electric Co.) 



HOW TO EXPERIMENT 



219 



115 shows how. You can measure the gain in audi- 
bility at the same time. 

After learning to use the audion as a detector, 
both by virtue of its curved characteristic and by the 
grid-condenser method, I would suggest studying 
the same tube as an amplifier. First I would learn 
to use it as an audio-frequency amplifier. Set up 
the crystal detector circuit. Use your audio-fre- 
quency transformer the other way around so as to 

s-yi/zTc^f TO cof^fOfiifE: 




^I'l'Hil'P- 



/=v^ //3 






step up to the grid. Put the telephone in the plate 
circuit. Choose your C-battery for amplification 
and not detection and try to receive. 

You will get better results if you can afford another 
iron-core transformer. If you can, buy one which 
will work between the plate circuit of one vacuum 
tube and the grid circuit of another similar tube. 
Then you will have the right equipment when you 
come to make a two-stage audio-frequency amplifier. 
If you buy such a transformer use the other trans- 
former between plate and telephones as you did be- 
fore and insert the new one as shown in Fig. 116. 



220 



LETTERS OF A RADIO-ENGINEER 



This circuit also shows how you can connect the 
switches so as to see how much the audion is ampli- 
fying. 

The next step is to use the audion as an amplifier 
of the radio-signal before its detection. Use the 
proper C-battery for an amplifier, as determined 



"//V' OF? *'OU-r" /=fC/0/B/LITir 




Your? ntrv^ 






riQ jje 



m 



from the blue print of the tube characteristic. Con- 
nect the tube as shown in Fig. 117. You will see 
that in this circuit we are using a choke coil to keep 
the radio-frequency current out of the battery part 
of the plate circuit and a small condenser, another 
one of 0.002 mf., to keep the battery current from 
the crystal detector. You can see from the same 
figure how you can arrange the swifches so as to 
find whether or not you are getting any gain from 
the amplifier. 



HOW TO EXPERIMENT 



221 



Now you are ready to receive those C-W senders 
at 275 meters. You will need to wind another coil 
like the secondary coil you already have. Here is 
where you buy another condenser. You will need it 
later. If before you bought the 0.0005 size, this time 
buy the 0.001 size or vice versa. "Wind also a small 
coil for a tickler. About 20 turns of 26 wire on a core 
of 3% in. diameter will do. Connect the tickler in 



G/9/rf /='/70r^ r=fFiOJO 




ri<p //? 



the plate circuit of the audion. Connect to the grid 
your new coil and condenser and set the audion 
circuit so that it will induce a current in the secon- 
dary circuit which supplies the crystal. Fig. 118 
shows the hook-up. 

You will see that you are now supplying the crys- 
tal with current from two sources, namely the dis- 
tant source of the incoming signals and the local 
oscillator which you have formed. The crystal will 
detect the *'beat note" between these two currents. 

To receive the 275 meters signals you will need to 
make several adjustments at the same time. In the 
first place I would set the tuning of the antenna 



222 LETTERS OF A RADIO-ENGINEER 



circuit and of the crystal circuit about where you 
think right because of your knowledge of the set- 
tings for other wave lengths. Then I would get the 
local oscillator going. You can tell whether or not 
it is going if you suddenly increase or decrease the 
coupling between the tickler coil and the input cir- 
cuit of the audion. If this motion is accompanied 
by a click in the receivers the tube is oscillating. 






^F^ 







Now you must change the frequency at which it 
is oscillating by slowly changing the capacity in the 
tuned input circuit of the tube. Unless the antenna 
circuit is properly tuned to the 275 meter signal 
you will get no results. If it is, you will hear an 
intermittent musical note for some tune of your 
local oscillator. This note will have the duration of 
dots and dashes. 

You will have to keep changing the tuning of your 
detector circuit and of the antenna. For each new 
setting very slowly swing the condenser plates in 
the oscillator circuit and see if you get a signal. It 



HOW TO EXPERIMENT 223 

will probably be easier to use the *^ stand-by posi- 
tion,'' which I have described, with switch S open 
in the secondary circuit of Fig. 118. In that case 
you have only to tune your antenna to 275 meters 
and then you will pick up a note when your local 
oscillator is in tune. After you have done so you 
can tune the secondary circuit which supplies the 
crystal. 

If you adopt this method you will want a close 
coupling between the antenna and the crystal cir- 
cuit. You will always want a very weak coupling 
between the oscillator circuit and the detector cir- 
cuit. You will also probably want a weaker coup- 
ling between tickler and tube input than you are 
at first inclined to believe will be enough. Patience 
and some skill in manipulation is always required 
for this sort of experiment. 

When you have completed this experiment in het- 
erodyne receiving, using a local oscillator, you are 
ready to try the regenerative circuit. This has been 
illustrated in Fig. 92 of Letter 18 and needs no fur- 
ther description. You will have the advantage when 
you come to this of knowing very closely the proper 
settings of the antenna circuit and the secondary 
tuned circuit. You will need then only to adjust the 
coupling of the tickler and make finer adjustments 
in your tuning. 

After you have completed this series of experi- 
ments you will be something of an adept at radio 
and are in a position to plan your final set. For 
this set you will need to purchase certain parts 



224 



LETTERS OF A RADIO-ENGINEER 



complete from reputable dealers because many of 
the circuits which I have described are patented 
and should not be used except as rights to use are 
obtained by the purchase of licensed apparatus 
which embodies the patented circuits. Knowing 
how radio receivers operate and why, you are now 
in a good condition to discuss with dealers the rela- 
tive merits and costs of receiving sets. 
Before you actually buy a completed set you may 



< 



lO/=fOJ/^($ 



YOU UV/Ll. f=>/=?OBtQeL^ 

/9LSO f^of? r/v<r co/yQETR 








rfq //^ 



want to increase the range of frequency over which 
you are carrying out your experiments. To receive 
at longer wave-lengths you will need to increase the 
inductance of your antenna so that it will be tuned 
to a lower frequency. This is usually called ^ load- 
ing" and can be done by inserting a coil in the an- 
tenna. To obtain smaller wave-lengths decrease the 
effective capacity of the antenna circuit by putting 
another condenser in series with the antenna. Usu- 
ally, therefore, one connects into his antenna circuit 
both a condenser and a loading coil. By using a var- 
iable condenser the effective capacity of the antenna 
system may be easily changed. The result is that this 



HOW TO EXPERIMENT 225 

series condenser method becomes the easiest method 
of tuning and the slide wire tuner is not needed. Fig. 
119 shows the circuit. 

For quite a range of wave-lengths we may use the 
same loading coil and tune the antenna circuit en- 
tirely by this series condenser. For some other 
range of wave-lengths we shall then need a different 
loading coil. In a well-designed set the wave-length 
ranges overlap. The calculation of the size of loading 
coil is quite easy but requires more arithmetic than I 
care to impose on you at present. I shall therefore 
merely give you illustrations based on the assump- 
tion that your antenna has a capacity of 0.0001 or of 
0.0002 mf. and that the condensers which you have 
bought are 0.0005 and 0.001 for their maxima. 

In Table I there is given, for each of several values 
of the inductance of the primary coil, the shortest 
and the longest wave-lengths which you can expect 
to receive. The table is in two parts, the first for 
an antenna of capacity 0.0001 mf. and the second 
for one of 0.0002 mf . Yours will be somewhere be- 
tween these two limits. The shortest wave-length 
depends upon the antenna and not upon the con- 
denser which you use in series with it for timing. 
It also depends upon how much inductance there is 
in the coil which you have in the antenna circuit. 
The table gives values of inductance in the first 
column, and of minimum waive-length in the second. 
The third column shows what is the greatest wave- 
length you may expect if you use a tuning condenser 
of 0.0005 mf. ; and the fourth column the slightly 



226 LETTERS OF A RADIO-ENGINEER 

large wave-length which is possible with the larger 
condenser. 







TABLE 


I 






Paet 1. (For antenna of 0.0001 mf.) 




ductance in 


Shortest wave 


-length 


Longest wave-length in met 


il-henries. 


in meters 




with 0.0005 mf. 


with 0.001 


0.10 


103 




169 


179 


0.20 


146 




238 


253 


0.40 


207 




337 


358 


0.85 


300 




490 


515 


1.80 


400 




700 


760 


2.00 


420 




750 


800 


4.00 


600 




1080 


1130 


5.00 


660 




1200 


1260 


10.00 


900 




1700 


1790 


30.00 


1600 




2900 


3100 




Paet 2. (For antenna of 0.0002 mf.) 




0.10 


169 




225 


240 


0.16 


210 




285 


305 


0.20 


240 




320 


340 


0.25 


270 




355 


380 


0.40 


340 




450 


480 


0.60 


420 




550 


590 


0.80 


480 




630 


680 


1.20 


585 




775 


840 


1.80 


720 




950 


1020 


3.00 


930 




1220 


1320 


5.00 


1200 




1600 


1700 


8.00 


1500 




2000 


2150 


12.00 


1850 




2400 


2650 


16.00 


2150 




2800 


3050 



From Table I you can find how much inductance 
you will need in the primary circuit. A certain 
amount you will need to couple the antenna and 
the secondary circuit. The coil which you wound 
at the beginning of your experiments will do well 
for that. Anything more in the way of inductance, 
which the antenna circuit requires to give a desired 
wave-length, you may consider as loading. In Table 
II are some data as to winding coils on straight 



HOW TO EXPERIMENT 227 

cores to obtain various values of inductance. Your 
26 s. s. c. wire will wind about 54 turns to the inch. 
I have assumed that you will have this number of 
turns per inch on your coils and calculated the in- 
ductance which you should get for various numbers 
of total turns. The first part of the table is for a 
core of S.5 inches in diameter and the second part 
for one of 5 inches. The first column gives the in- 
ductance in mil-henries. The second gives number 
of turns. The third and fourth are merely for con- 
venience and give the approximate length in inches 
of the coil and the approximate total length of wire 
which is required to wind it. I have allowed for 
bringing out taps. In other words 550 feet of the 
wire will wind a coil of 10.2 inches with an induct- 
ance of 8.00 mil-henries, and permit you to bring 
out taps at all the lower values of inductance which 
are given in the table. 





Paet 1. (For a core of 3.5 in. diam.) 




Inductance in 


Number 


Length 


Feet of wire 


mil-henries. 


of turns. 


in inches. 


required. 


0.10 


25 


0.46 


25 


0.16 


34 


0.63 


36 


0.20 


39 


0.72 


42 


0.25 


44 


0.81 


49 


0.40 


58 


1.07 


63 


0.60 


75 


1.38 


80 


0.80 


92 


1.70 


100 


0.85 


96 


1.78 


104 


1.00 


108 


2.00 


118 


1.20 


123 


2.28 


133 


1.80 


164 


3.03 


176 


2.00 


180 


3.33 


190 


3.00 


242 


4.48 


250 


4.00 


304 


6.62 


310 


5.00 


366 


6.77 


370 


8.00 


550 


10.20 


550 



I 



228 LETTERS OF A RADIO-ENGINEER 

Paet 2. (For core of 5.0 in. diam.) 



2.00 


120 


2.22 


160 


3.00 


158 


2.93 


215 


4.00 


194 


3.58 


265 


5.00 


228 


4.22 


310 


8.00 


324 


6.00 


450 


10.00 


384 


7.10 


530 


12.00 


450 


8.30 


625 



II 



The coil whicli you wound at the beginning of your 
experiment had only 75 turns and was tapped so that 
you could, by manipulating the two switches of Fig. 
112, get small variations in inductance. In Table 
III is given the values of the inductance which is 
controlled by the switches of that figure, the corre- 
sponding number of turns, and the wave-length to 
which the antenna should then be tuned. I am giv- 
ing this for two values of antenna capacity, as I 
have done before. By the aid of these three tables 
you should have small difficulty in taking care of 
matters of tuning for all wave-lengths below about 
3000 meters. If you want to get longer waves than 
that you had better buy a few banked-wound coils. 
These are coils in which the turns are wound over 
each other but in such a way as to avoid in large part 
the ^^ capacity effects" which usually accompany such 
winding. You can try winding them for yourself 
but I doubt if the experience has much value until 
you have gone farther in the study of the mathe- 
matical theory of radio than this series of letters will 
carry you. 



HOW TO EXPERIMENT 229 





TABLE 


III 












Circuit of 


Fiff. 


112 


Number 


Inductance in 


Wave length 


with antenna of 


of turns. 


mil-henries. 


0.0001 mf. 




0.0002 mf . 


14 


0.04 


120 






170 


20 


0.07 


160 






220 


28 


0.12 


210 






290 


36 


0.18 


250 






360 


44 


0.25 


300 






420 


56 


0.38 


370 






520 


75 


0.60 


460 






650 



In the secondary circuit there is only one capacity, 
that of the variable condenser. If it has a range of 
values from about 0.00005 mf . to 0.0005 mf . your coil 
of 60 turns and 0.42 mf. permits a range of wave- 
lengths from 270 to 860 m. Using half the coil the 
range is 150 to 480 m. With the larger condenser 
the ranges are respectively 270 to 1220 and 270 to 
670. For longer wave-lengths load with inductance. 
Four times the inductance will tune to double these 
wave-lengths. 



LETTER 22 

HIGH-POWERED RADIO-TELEPHONE 
TRANSMITTERS 

My Deak Expeeimenter : 

This letter is to summarize the operations which 
must be performed in radio-telephone transmission 
and reception ; and also to describe the circuit of an 
important commercial system. 

To transmit speech by radio three operations are 
necessary. First, there must be generated a high- 
frequency alternating current; second, this current 
must be modulated, that is, varied in intensity in ac- 
cordance with the human voice ; and third, the modu- 
lated current must be supplied to an antenna. For 
efficient operation, of course, the antenna must be 
tuned to the frequency which is to be transmitted. 
There is also a fourth operation which is usually 
performed and that is amplification. Wherever the 
electrical effect is smaller than desired, or required 
for satisfactory transmission, vacuum tubes are used 
as amplifiers. Of this I shall give you an illustra- 
tion later. 

Three operations are also essential in receiving. 
First, an antenna must be so arranged and tuned 
as to receive energy from the distant transmitting 
station. There is then in the receiving antenna a 
current similar in wave form to that in the trans- 

230 



HIGH-POWERED TRANSMITTERS 231 

mitting antenna. Second, the speech significance of 
this current must be detected, that is, the modulated 
current must be demodulated. A current is then 
obtained which has the same wave form as the human 
voice which was the cause of the modulation at the 
distant station. The third operation is performed 
by a telephone receiver which makes the molecules 
of air in its neighborhood move back and forth in 
accordance with the detected current. As you al- 
ready know a ^fourth operation may be carried on 
by amplifiers which give on their output sides cur- 
rents of greater strength but of the same forms 
as they receive at their input terminals. 

In transmitting and in receiving equipment two 
or more of these operations may be performed by 
the same vacuum tube as you will remember from 
our discussion of the regenerative circuit for re- 
ceiving. For example, also, in any receiving set 
the vacuum tube which detects is usually amplify- 
ing. In the regenerative circuit for receiving con- 
tinuous waves by the heterodyne method the vacuum 
tube functions as a generator of high-frequency 
current and as a detector of the variations in cur- 
rent which occur because the locally-generated cur- 
rent does not keep in step with that generated at 
the transmitting station. 

Another example of a vacuum tube performing 
simultaneously two different functions is illustrated 
in Fig. 120 which shows a simple radio-telephone 
transmitter. The single tube performs in itself both 
the generation of the radio-frequency current and its 



232 LETTERS OF A RADIO-ENGINEER 

modulation in accordance with the output of the car- 
bon-button transmitter. This audion is in a feed- 
back circuit, the oscillation frequency of which de- 
pends upon the condenser C and the inductance L. 
The voice drives the diaphragm of the transmitter 
and thus varies the resistance of the carbon button. 
This varies the current from the battery, Ba, through 
the primary, Ji, of the transformer T. The result 
is a varying voltage applied to the grid by the secon- 



se?* 




r/q/ao 



dary T2. The* oscillating current in the plate circuit 
of the audion varies accordingly because it is de- 
pendent upon the grid voltage. The condenser Cr 
offers a low impedance to the radio-frequency current 
to which the winding T2 of audio-frequency trans- 
former offers too much. 

In this case the tube is both generator and *' mod- 
ulator. '^ In some cases these operations are sep- 
arately performed by different tubes. This was true 
of the transmitting* set used in 1915 when the engi- 
neers of the Bell Telephone System talked by radio 
from Arlington, near Washington, D. C, to Paris 
and Honolulu. I shall not draw out completely the 
circuit of their apparatus but I shall describe it by 



HIGH-POWERED TRANSMITTERS 



233 



using little squares to represent the parts respon- 
sible for each of the several operations. 

First there was a vacuum tube oscillator which 
generated a small current of the desired frequency. 
Then there was a telephone transmitter which made 
variations in a direct-current flowing through the 
primary of a transformer. The e. m. f. from the 
secondary of this transformer and the e. m. f . from 
the radio-frequency oscillator were both impressed 
upon the grid of an audion which acted as a modu- 




•1^ 



lator. The output of this audion was a radio-fre- 
quency current modulated by the voice. The output 
was amplified by a two-stage audion amplifier and 
supplied through a coupling coil to the large antenna 
of the U. S. Navy Station at Arlington. Fig. 121 
shows the system. 

The audion amplifiers each consisted of a number 
of tubes operating in parallel. "When tubes are op- 
erated in parallel they are connected as shown in 
Fig. 122 so that the same e. m. f. is impressed on 
all the grids and the same plate-battery voltage on 
all the plates. As the grids vary in voltage there is 
a corresponding variation of current in the plate 
circuit of each tube. The total change of the cur- 



234 LETTERS OF A RADIO-ENGINEER 



rent in the plate-battery circuit is, then, the sum of 
the changes in all the plate-filament circuits of the 
tubes. This scheme of connections gives a result 
equivalent to that of a single tube with a correspond- 
ingly larger plate and filament. 

Parallel connection is necessary because a single 
tube would be overheated in delivering to the an- 







the: /=-/L/?r*f£-r^rs Rf?E: 

USURLLY CONnCCTEO 
in RRRRLUETL A!?/yO 

tenna the desired amount of power. You remember 
that when the audion is operated as an amplifier 
the resistance to which it supplies current is made 
equal to its own internal resistance of Ep. That 
means that there is in the plate circuit just as much 
resistance inside the tube as outside. Hence there is 
the same amount of work done each second in forcing 
the current through the tube as through the antenna 
circuit, if that is what the tube supplies. ''Work 
per second'* is power; the plate battery is spending 




Pl. XI. — Broadcasting Equipment, Developed by the American 
Telephone and Telegraph Company and the Western Electric 
Company. 



HIGH-POWERED TRANSMITTERS 235 

energy in the tube at the same rate as it is supply- 
ing it to the antenna where it is useful for radiation. 

All the energy expended in the tube appears as 
heat. It is due to the blows which the electrons strike 
against the plate when they are drawn across from 
the filament. These impacts set into more rapid mo- 
tion the molecules of the plate ; and the temperature 
of the tube rises. There is a limit to the amount 
the temperature can rise without destroying the tube. 
For that reason the heat produced inside it must 
not exceed a certain limit depending upon the de- 
sign of the tube and the method of cooling it as it is 
operated. In the Arlington experiments, which I 
mentioned a moment ago, the tubes were cooled by 
blowing air on them from fans. 

We can find the power expended in the plate cir- 
cuit of a tube by multiplying the number of volts 
in its battery by the number of amperes which flows. 
Suppose the battery is 250 volts and the current 
0.02 amperes, then the power is 5 watts. The ^ ' watt ' ' 
is the unit for measuring power. Tubes are rated 
by the number of watts which can be safely ex- 
pended in them. You might ask, when you buy an 
audion, what is a safe rating for it. The question 
will not be an important one, however, unless you are 
to set up a transmitting set since a detector is usu- 
ally operated with such small plate-voltage as not 
to have expended in it an amount of power dangerous 
to its life. 

In recent transmitting sets the tubes are used in 
parallel for the reasons I have just told, but a dif- 



236 LETTERS OF A RADIO-ENGINEER 

ferent method of modulation is used. The genera- 
tion of the radio-frequency current is by large-pow- 
ered tubes which are operated with high voltages 
in their plate circuits. The output of these oscil- 
lators is supplied to the antenna. The intensity of 
the oscillations of the current in these tubes is con- 
trolled by changing the voltage applied in their plate 
circuits. You can see from Fig. 123 that if the plate 
voltage is changed the strength of the alternating 







L H M L 

current is changed accordingly. It is the method 
used in changing the voltage which is particularly 
interesting. 

The high voltages which are used in the plate cir- 
cuits of these high-powered audions are obtained 
from generators instead of batteries. You remember 
from Letter 20 that an e. m. f. is induced in a coil 
when the coil and a magnet are suddenly changed 
in their positions, one being turned with reference 
to the other. A generator is a machine for turning 
a coil so that a magnet is always inducing an e. m. f. 
in it. It is formed by an armature carrying coils and 
by strong electromagnets. The machine can be 



HIGH-POWERED TRANSMITTERS 



237 







r/f£ ClSC/LL/?rOR 



f7^/a^ 






driven by a steam or gas engine, by a water wheel, 
or by an electric motor. Generators are designed 
either to give steady streams of electrons, that is for 
d-c currents, or to act as alternators. 

Suppose we have, as shown 
in Fig. 124, a d-c generator 
supplying current to a vac- 
uum tube oscillator. The 
current from the generator 
passes through an iron-cored 
choke coil, marked La in the 
figure. Between this coil and 
the plate circuit we connect 
across the line a telephone 
transmitter. To make a sys- 
tem which will work effi- 
ciently we shall have to suppose that this trans- 
mitter has a high resistance, say about the same as 
the internal resistance, i?p, of the tube and also that 
it can carry as large a current. 

Of the current which comes from the generator 
about one-half goes to the tube and the rest to the 
transmitter. If the resistance of the transmitter 
is increased it can^t take as much current. The coil, 
La, however, because of its inductance, tends to keep 
the same amount of current flowing through itself. 
For just an instant then the current in La keeps 
steady even though the transmitter doesn't take its 
share. The result is more current for the oscillat- 
ing tube. On the other hand if the transmitter takes 
more current, because its resistance is decreased, 



238 LETTERS OF A RADIO-ENGINEER 

the choke coil, La, will momentarily tend to keep 
the current steady so that what the transmitter takes 
mnst be at the expense of the oscillating tnbe. 

That's one way of looking at what happens. "We 
know, however, from Fig. 123 that to get an in- 
crease in the amplitude of the current in the oscillat- 
ing tube w^e must apply an increased voltage to its 
plate circuit. That is what really happens when 
the transmitter increases in resistance and so doesn't 
take its full share of the current. The reason is 
this : When the transmitter resistance is increased 
the current in the transmitter decreases. Just for 
a moment it looks as though the current in La is 
going to decrease. That's the way it looks to the 
electrons ; and you know what electrons do in an in- 
ductive circuit when they think they shall have to 
stop. They induce each other to keep on for a mo- 
ment. For a moment they act just as if there was 
some extra e. m. f. which was acting to keep them 
going. We say, therefore, that there is an extra 
e. m. f ., and we call this an e. m. f. of self-induction. 
All this time there has been active on the plate circuit 
of the tube the e. m. f. of the generator. To this 
there is added at the instant when the transmitter 
resistance increases, the e. m. f . of self-induction in 
the coil, La; and so the total e. m. f. applied to the 
tube is momentarily increased. This increased e. 
m. f ., of course, results in an increased amplitude for 
the alternating current which the oscillator is supply- 
ing to the transmitting antenna. 

When the transmitter resistance is decreased, and 



HIGH-POWERED TRANSMITTERS 



239 



a larger current should flow through the choke coil, 
the electrons are asked to speed up in going through 
the coil. At first they object and during that instant 
they express their objection by an e. m. f. of self- 
induction which opposes the generator voltage. For 
an instant, then, the voltage of the oscillating tube 



^/7D£: PfiFlT or THE TU/ieo 




^NflMRUr/€R so TH/^T 
T/i£: T£-L€PHOrnr M/9Y 

/=>ffOOuc£: L. fi/fo^ CH^n^cs 

4ryTi-fe /^irsjs-r/=fnc£:of^ 

THE' r^ooui^/^TOf^ yc c^Ho^rs: co/L 

is lowered and its alternating-current output is 
smaller. 

For the purpose of bringing about such threatened 
changes in current, and hence such e. m. f . 's of self- 
induction, the carbon transmitter is not suitable be- 
cause it has too small a resistance and too small a 
current carrying ability. The plate circuit of a 
vacuum tube will serve admirably. You know from 
the audion characteristic that without changing the 
plate voltage we can, by applying a voltage to the 
grid, change the current through the plate circuit. 



240 LETTERS OF A RADIO-ENGINEER 

Now if it was a wire resistance with which we 
were dealing and we should be able to obtain a 
change in current without changing the voltage 
acting on this wire we would say that we had 
changed the resistance. We can say, therefore, that 
the internal resistance of the plate circuit of a 
vacuum tube can be changed by what we do to the 
grid. 

In Fig. 125 I have substituted the plate circuit of 
an audion for the transmitter of Fig. 124 and 
arranged to vary its resistance by changing the po- 
tential of the grid. This we do by impressing upon 
the grid the e. m. f. developed in the secondary of 
a transformer, to the primary of which is connected a 
battery and a carbon transmitter. The current 
through the primary varies in accordance with the 
sounds spoken into the transmitter. And for all the 
reasons which we have just finished studying there 
are similar variations in the output current of the 
oscillating tube in the transmitting set of Fig. 125. 

In this latter figure you will notice a small air- 
core coil. Lb, between the oscillator and the modula- 
tor tube. This coil has a small inductance but it is 
enough to offer a large impedance to radio-frequency 
currents. The result is, it does not let the alternating 
currents of the oscillating tube flow into the modu- 
lator. These currents are confined to their own cir- 
cuit, where they are useful in establishing similar 
currents in the antenna. On the other hand, the 
coil Lb doesn't seriously impede low-frequency cur- 
rents and therefore it does not prevent variations 



HIGH-POWERED TRANSMITTERS 241 

in the current wMch are at audio-frequency. It does 
not interfere with the changes in current which ac- 
company the variations in the resistance of the plate 
circuit of the modulator. That is, it has too little 
impedance to act like La and so it permits the modu- 
lator to vary the output of the oscillator. 




Ct /s the: Tc//y/r-/G co/yas'/v^sr/^ 

C//=i'CC//T ^HO\rV^ THfilT TMST COU/°L/rv<? 

The oscillating circuit of Fig. 125 includes part of 
the antenna. It differs also from the others I have 
shown in the manner in which grid and plate circuits 
are coupled. I'll explain by Fig. 126. 

The transmitting set which I have just described 
involves many of the principles of the most modern 
sets. If you understand its operation you can prob- 
ably reason out for yourself any of the other sets 
of which you will hear from time to time. 



LETTER 23 

A]MPLIFICATION AT INTERMEDIATE 
FREQUENCIES 

Deae Sox: 

111 tlie matter of receiving I have already covered 
all the important principles. There is one more 
system, however, which you will need to know. This 
is spoken of either as the '^super-heterodyne'' or as 
the ''intermediate-frequency amplification'' method 
of reception. 

The system has two important advantages. First, 
it permits sharper tuning and so reduces interfer- 
ence from other radio signals. Second, it permits 
more amplification of the incoming signal than is 
usually practicable. 

First as to amplification: We have seen that am- 
plification can be accomplished either by amplifying 
the radio-frequency current before detection or by 
amplifying the audio-frequency current which re- 
sults from detection. There are practical limita- 
tions to the amount of amplification which can be 
obtained in either case. An efficient multi-stage 
amplifier for radio-frequencies is difficult to build 
because of what we call "capacity effects." 

Consider for example the portion of circuit shown 
in Fig. 127. The wires a and h act like small plates 
of condensers. What we really have, is a lot of 

242 



INTERMEDIATE FREQUENCIES 243 

tiny condensers which I have shown in the figure by 
the light dotted-lines. If the wires are transmitting 
high-frequency currents these condensers offer tiny 
waiting-rooms where the electrons can run in and 
out without having to go on to the grid of the next 
tube. There are other difficulties in high-frequency 
amplifiers. This one of capacity effects between 
parallel wires is enough for the present. It is per- 
haps the most interesting because it is always more 



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or less troublesome whenever a pair of wires is used 
to transmit an alternating current. 

In the case of a multi-stage amplifier of audio- 
frequency current there is always the possibility 
of the amplification of any small variations in cur- 
rent which may naturally occur in the action of the 
batteries. There are always small variations in the 
currents from batteries, due to impurities in the 
materials of the plates, air bubbles, and other causes. 
Ordinarily we don't observe these changes because 
they are too small to make an audible sound in the 
telephone receivers. Suppose, however, that they 
take place in the battery of the first tube of a series 
of amplifiers. Any tiny change of current is ampli- 
fied many times and results in a troublesome noise in 



244 LETTERS OF A RADIO-ENGINEER 

the telephone receiver which is connected to the last 
tube. 

In both types of amplifiers there is, of course, al- 
ways the chance that the output circuit of one tube 
may be coupled to and induce some effect in the 
input circuit of one of the earlier tubes of the series. 
This will be amplified and result in a greater induc- 
tion. In other words, in a circuit where there is large 
amplification, there is always the difficulty of avoid- 
ing a feed-back of energy from one tube to another 
so that the entire group acts like an oscillating cir- 
cuit, that is " regeneratively. ' ' Much of this diffi- 
culty can be avoided after experience. 

If a multi-stage amplifier is to be built for a cur- 
rent which does not have too high a frequency the 
'' capacity effects" and the other difficulties due to 
high-frequency need not be seriously troublesome. 
If the frequency is not too high, but is still well above 
the audible limit, the noises due to variations in bat- 
tery currents need not bother for they are of quite 
low frequency. Currents from 20,000 to 60,000 
cycles a second are, therefore, the most satisfactory 
to amplify. 

Suppose, however, one wishes to amplify the sig- 
nals from a radio-broadcasting station. The w^ave- 
length is 360 meters and the frequency is about 
834,000 cycles a second. The system of intermediate- 
frequency amplification solves the difficulty and we 
shall see how it does so. 

At the receiving station a local oscillator is used. 
This generates a frequency which is about 30,000 



INTERMEDIATE FREQUENCIES 245 

cycles less than that of the incoming signal. Both 
currents are impressed on the grid of a detector. 
The result is, in the output of the detector, a current 
which has a frequency of 30,000 cycles a second. 







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The intensity of this detected current depends upon 
the intensity of the incoming signal. The ''beat 
note" current of 30,000 cycles varies, therefore, in 
accordance with the voice which is modulating at 
the distant sending station. The speech significance 
is now hidden in a current of a frequency interme- 
diate between radio and audio. This current may 



246 LETTERS OF A RADIO-ENGINEER 

be amplified many times and then supplied to the 
gi-id of a detector which obtains from it a current 
of audio-frequency which has a speech significance. 
In Fig. 128 I have indicated the several operations. 
We can now see why this method permits sharper 
tuning. The whole idea of tuning, of course, is to 
arrange that the incoming signal shall cause the 
largest possible current and at the same time to pro- 
vide that any signals at other wave-lengths shall 
cause only negligible currents. What we want a 
receiving set to do is to distinguish between two 
signals which differ slightly in wave-length and to 
respond to only one of them. 

Suppose we set up a tuned circuit formed by a 
coil and a condenser and try it out for various fre- 
quencies of signals. You know how it will respond 
from our discussion in connection with the tuning 
curve of Fig. 51 of Letter 13. We might find from 
a number of such tests that the best we can expect 
any tuned circuit to do is to discriminate between 
signals which differ about ten percent in frequency, 
that is, to receive well the desired signal and to 
fail practically entirely to receive a signal of a fre- 
quency either ten percent higher or the same amount 
lower. 

For example, if the signal is at 30,000 cycles a 
tuned circuit might be expected to discriminate 
against an interfering signal of 33,000. If the signal 
is at 300,000 cycles a tuned circuit might discrimi- 
nate against an interfering signal of 330,000 cycles, 
but an interference at 303,000 cycles would be very 



INTERMEDIATE FREQUENCIES 247 

troublesome indeed. It couldn't be *' tuned ouf at 
all. 

Now suppose that the desired signal is at 300,000 
cycles and that there is interference at 303,000 cycles. 
We provide a local oscillator of 270,000 cycles a 
second, receive by this ^'super-heterodyne'' method 
which I have just described, and so obtain an inter- , 
mediate frequency. In the output of the first detec- 
tor we have then a current of 300,000-270,000 or 
30,000 cycles due to the desired signal and also a cur- 
rent of 303,000-270,000 or 33,000 cycles due to the 
interference. Both these currents we can supply 
to another tuned circuit which is tuned for 30,000 
cycles a second. It can receive the desired signal 
but it can discriminate against the interference be- 
cause now the latter is ten percent *'off the tune" 
of the signal. 

You see the question is not one of how far apart 
two signals are in number of cycles per second. The 
question always is : How large in percent is the dif- 
ference between the two frequencies? The matter of 
separating two effects of different frequencies is a 
question of the 'interval'* between the frequencies. 
To find the interval between two frequencies we 
divide one by the other. You can see that if the 
quotient is larger than 1.1 or smaller than 0.9 the 
frequencies differ by ten percent or more. The 
higher the frequency the larger the number of cycles 
which is represented by a given size of interval. 

While I am writing of frequency intervals I want 
to tell you one thing more of importance. You re- 



24^8 LETTERS OF A RADIO-ENGINEER 

member that in human speech there may enter, and 
be necessary, any frequency between about 200 and 
2000 cycles a second. That we might call the range 
of the necessary notes in the voice. Whenever we 
want a good reproduction of the voice we must re- 
produce all the frequencies in this range. 

Suppose we have a radio-current of 100,000 cycles 
modulated by the frequencies in the voice range. We 
find in the output of our transmitting set not only a 
current of 100,000 cycles but currents in two other 
ranges of frequencies. One of these is above the 
signal frequency and extends from 100,200 to 102,000 
cycles. The other is the same amount below and 
extends from 98,000 to 99,800 cycles. We -say there 
is an upper and a lower '^band of frequencies.'^ 

All these currents are in the complex wave which 
comes from the radio-transmitter. For this state- 
ment you will have to take my word until you can 
handle the form of mathematics known as ' ^ trigono- 
metry. ' ' When we receive at the distant station we 
receive not only currents of the signal frequency 
but also currents whose frequencies lie in these 
^* side-bands. '* 

No matter what radio-frequency we may use we 
must transmit and receive side-bands of this range 
if we use the apparatus I have described in the past 
letters. You can see what that means. Suppose we 
transmit at a radio-frequency of 50,000 cycles and 
modulate that with speech. We shall really need 
all the range from 48,000 cycles to 52,000 cycles for 
one telephone message. On the other hand if we 



INTERMEDIATE FREQUENCIES 249 

modulated a 500,000 cycle wave by speech the side- 
bands are from 498,000 to 499,800 and 500,200 to 
502,000 cycles. If we transmit at 50,000 cycles, that 
is, at 6000 meters, we really need all the range be- 
tween 5770 meters and 6250 meters, as you can see 
by the frequencies of the side bands. At 100,000 
cycles we need only the range of wave-lengths be- 
tween 2940 m. and 3060 m. If the radio-frequency 
is 500,000 cycles we need a still smaller range of 
wave-lengths to transmit the necessary side bands. 
Then the range is from 598 m. to 603 m. 

In the case of the transmission of speech by radio 
we are interested in having no interference from 
other signals which are within 2000 cycles of the 
frequency of our radio-current no matter what their 
wave-lengths may be. The part of the wave-length 
range which must be kept clear from interfering 
signals becomes smaller the higher the frequency 
which is being modulated. 

You can see that very few telephone messages 
can be sent in the long-wave-length part of the radio 
range and many more, although not very many after 
all, in the short wave-length part of the radio range. 
You can also see why it is desirable to keep ama- 
teurs in the short wave-length part of the range 
where more of them can transmit simultaneously 
without interfering with each other or with com- 
mercial radio stations. 

There is another reason, too, for keeping amateurs 
to the shortest wave-lengths. Transmission of radio 
signals over short distances is best accomplished by 



250 LETTERS OF A RADIO-ENGINEER 



short wave-lengths but over long distances by the 
longer wave-lengths. For trans-oceanic work the 
very longest wave-lengths are best. The '' long- 
haul '^ stations, therefore, work in the frequency 
range immediately above 10,000 cycles a second and 
transmit with wave lengths of 30,000 m. and shorter. 




Pl. XII. — Broadcasting Station of the xA-Imerican Telephone 
AND Telegraph Company on the Roof of the \Valker-Lispex.vrd 
Bldg. in New York City Where the Long-distance Telephone 
Lines Terminate. 






LETTER 24 
BY WIRE AND BY RADIO 

Deae Boy : 

The simplest wire telephone-circuit is formed by a 
transmitter, a receiver, a battery, and the connecting 

wire. If two persons are to 
carry on a conversation each 
mnst have this amount of 

n^^_^,^ —/\^ equipment. The apparatus 

"U^ I fAJp^ might be arranged as in Fig. 

H|J||]h 129. This set-up, however, 

^ /;?Q requires four wires between 

the two stations and you 
know the telephone company uses only two wires. 
Let us find the principle upon which its system 
operates because it is the solution of many different 
problems including that of wire- 
to-radio connections. 

Imagine four wire resistances 
connected together to form a 
square as in Fig. 130. Suppose 
there are two pairs of equal re- 
sistances, namely J^i and Rz, and 
Zi and Zz. If we connect a gen- 
erator, (t, between the junctions 
a and h there will be two separate streams of elec- 
trons, one through the R-side and the other through 

251 




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252 LETTERS OF A RADIO-ENGINEER 

the Z-side of the circuit. These streams, of course, 
will not be of the same size for the larger stream 
will flow through the side which offers' the smaller 
resistance. 

Half the e. m. f . between a and h is used up in send- 
ing the stream half the distance. Half is used be- 
tween a and the points c and d, and the other half 
between c and d and the other end. It doesn't make 
any difference whether we follow the stream from a 
to G or from a to d, it takes half the e. m. f. to keep 
this stream going. Points c and d, therefore, are in, 
the same condition of being ''half-way electrically" 
from a to h. The result is that there can be no cur- 
rent through any wire which we connect between c 
and d. 

Suppose, therefore, that we connect a telephone 
receiver between c and d. No current flows in it-and 
no sound is emitted by it. Now suppose the resist- 
ance of Z2 is that of a telephone line which stretches 
from one telephone station to another. Suppose also 
that Zi is a telephone line exactly like Z^ except that 
it doe§n't go anywhere at all because it is all shut 
up in a little box. We'll calLr^i^an artificial tele- 
phone line. We ought to call it, as little children 
would say, a ''make-believe" telephone line. It 
doesn't fool us but it does fool the electrons for they 
can't tell the difference between the real line Z. and 
the artificial line Z^. We can make a very good arti- 
ficial line by using a condenser and a resistance. The 
condenser introduces something of the capacity ef- 



BY WIRE AND BY RADIO 



253 



fects which I told you were always present in a 
circuit formed by a pair of wires. 

At the other telephone station let us duplicate 
this apparatus, using the same real line in both cases. 
Instead of just any generator of an alternating e. m. 
f. let us use a telephone transmitter. "We connect 
the transmitter through a transformer. The system 
then looks like that of Fig. 131. WTien some one talks 
at station 1 there is no current through his receiver 




(=^V.y<^ 



because it is connected to c and d, while the e. m. f. 
of the transmitter is applied to a and h,. The trans- 
mitter sets up two electron streams between a and h, 
and the stream which flows through the Z-side of 
the square goes out to station 2. At this station the 
electrons have three paths between d and J). I have 
marked these by arrows and you see that one of them 
is through the receiver. The current which is started 
by the transmitter at station 1 will therefore operate 
the receiver at station 2 but not at its own station. 
Of course station 2 can talk to 1 in the same way. 
The actual set-up used by the telephone company 



254 LETTERS OF A RADIO-ENGINEER 



is a little different from that which I have shown be- 
cause it nses a single common battery at a central 
office between two subscribers. The general prin- 
ciple, however, is the same. 

It won't make any differ- 
ence if we use equal induc- 
tance coils, instead of the 
R-resistances, and connect 
the transmitter to them in- 
ductively as shown in Fig. 
132. So far as that is con- 
cerned we can also use a 
transformer between the receiver and the points c 
and d, as shown in the same figure. 

We are now ready to put in radio equipment at 
station 2. In place of the telephone receiver at sta- 




ngi3Z 




tion 2 we connect a radio transmitter.'* Then what- 
ever a person at station 1 says goes by wire to 2 and 
on out by radio. In place of the telephone trans- 



BY WIRE AND BY RADIO 255 

mitter at station 2 we connect a radio receiver. 
Whatever that receives by radio is detected and goes 
by wire to the listener at station 1. In Fig. 133 I 
have shown the equipment of station 2. There you 
have the connections for wire to radio and vice versa. 

One of the most interesting developments of recent 
years is that of ^^ wired wireless" or ^''carrier-cur- 
rent telephony'^ over wires. Suppose that instead of 
broadcasting from the antenna at station 2 we ar- 
range to have its radio transmitter supply current 
to a wire circuit. We use this same pair of wires 
for receiving from the distant station. We can do 
this if we treat the radio transmitter and receiver 
exactly like the telephone instruments of Fig. 132 
and connect them to a square of resistances. One 
of these resistances is, of course, the line between 
the stations. I have shown the general arrangement 
in Fig. 134. 

You see what the square of resistances, or 
*^ bridge'' really does for us. It lets us use a single 
pair of wires for messages whether they are coming 
or going. It does that because it lets us connect a 
transmitter and also a receiver to a single pair of 
wires in such a way that the transmitter can't affect 
the receiver. Whatever the transmitter sends out 
goes along the wires to the distant receiver but 
doesn't affect the receiver at the sending station. 
This bridge permits this whether the transmitter and 
receiver are radio instruments or are the ordinary 
telephone instruments. 



T 



256 LETTERS OF A RADIO-ENGINEER 



By its aid we may send a modulated high-fre- 
quency current over a pair of wires and receive 
from the same pair of wires the high-frequency cur- 



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rent which is generated and modulated at the dis- 
tant end of the line. It lets us send and receive over 
the same pair of wires the same sort of a modulated 
current as we would supply to an antenna in radio- 



BY WIRE AND BY RADIO 257 

telephone transmitting. It is the same sort of a cur- 
rent but it need not be anywhere near as large be- 
cause we aren't broadcasting; we are sending di- 
rectly to the station of the other party to our con- 
versation. 

If we duplicate the apparatus we can use the same 
pair of wires for another telephone conversation 
without interfering with the first. Of course, we 
have to use a different frequency of alternating cur- 
rent for each of the two conversations. We can send 
these two different modulated high-frequency cur- 
rents over the same pair of wires and separate them 
by tuning at the distant end just as well as we do in 
radio. I won't sketch out for you the tuned circuits 
by which this separation is made. It's enough to 
give you the idea. 

In that way, a single pair of wires can be used 
for transmitting, simultaneously and without any in- 
terference, several different telephone conversations. 
It takes very much less power than would radio 
transmission and the conversations are secret. The 
ordinary telephone conversation can go on at the 
same time without any interference with those which 
are being carried by the modulations in high-fre- 
quency currents. A total of five conversations over 
the same pair of wires is the present practice. 

This method is used between many of the large 
cities of the U. S. because it lets one pair of wires 
do the work of five. That means a saving, for copper 
wire costs money. Of course, all the special appar- 
atus also costs money. You can see, therefore, that 



258 LETTERS OF A RADIO-ENGINEER 

this method wouldn't be economical between cities 
very close together because all that is saved by not 
having to buy so much wire is spent in building 
special apparatus and in taking care of it after- 
wards. For long lines, however, by not having to 
buy five times as much wire, the Bell Company saves 
more than it costs to build and maintain the extra 
special apparatus. 

I implied a moment ago why this system is called 
a '^carrier-current'' system; it is because "the high- 
frequency currents carry in their modulations the 
speech significance." Sometimes it is called a sys- 
tem of "multiplex" telephony because it permits 
more than one message at a time. 

This same general principle is also applied to the 
making of a multiplex system of telegraphy. In the 
multiplex telephone system we pictured transmitting 
and receiving sets very much like radio-telephone 
sets. If instead of transmitting speech each trans- 
mitter was operated as a C-W transmitter then it 
would transmit telegraph messages. In the same 
frequency range there can be more telegraph systems 
operated simultaneously without interfering with 
each other, for you remember how many cycles each 
radio-telephone message requires. For that reason 
the multiplex telegraph system which operates by 
carrier-currents permits as many as ten different 
telegraph messages simultaneously. 

You remember that I told you how capacity effects 
rob the distant end of a pair of wires of the alternat- 
ing current which is being sent to them. That is 



BY WIRE AND BY RADIO 



259 



always true but the effect is not very great unless the 
frequency of the alternating current is high. It's 
enough, however, so that every few hundred miles it 
is necessary to connect into the circuit an audion 



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amplifier. This is true of carrier currents especially, 
but also true of the voice-frequency currents of ordi- 
nary telephony. The latter, however, are not weak- 
ened, that is, ** attenuated," as much and conse- 
quently do not need to be amplified as much to 
give good intelligibility at the distant receiver. 

In a telephone circuit over such a long distance as 
from New York City to San Francisco it is usual to 
insert amplifiers at about a dozen points along the 
route. Of course, these amplifiers must work for 



260 LETTERS OF A RADIO-ENGINEER 




BY WIRE AND BY RADIO 261 

transmission in either direction, amplifying speech 
on its way to San Francisco or in the opposite direc- 
tion. At each of the amplifying stations, or *^ re- 
peater stations," as they are usually called, two 
vacuum tube amplifiers are used, one for each direc- 
tion. To connect these with the line so that each may 
work in the right direction there are used two of 
the bridges or resistance squares. You can see from 
the sketch of Fig. 135 how an alternating current 
from the east will be amplified and sent on to the 
west, or vice versa. 

There are a large number of such repeater stations 
in the United States along the important telephone 
routes. In Fig. 136 I am showing you the location 
of those along the route of the famous '^transcon- 
tinental telephone-circuit.'' This shows also a radio- 
telephone connection between the coast of California 
and Catalina Island. Conversations have been held 
between this island and a ship in the Atlantic Ocean, 
as shown in the sketch. The conversation was made 
possible by the use of the vacuum tube and the bridge 
circuit. Part of the way it was by wire and part by 
radio. "Wire and radio tie nicely together because 
both operate on the same general principles and use 
much of the same apparatus. 



INDEX 



A-battery for tubes, 42 

Accumulator, 29 

Acid, action of hydrogen in, 7 

Air, constitution of, 10 

Ammeter, alternating current, 
206; calibration of, 53; con- 
struction of, 205 

Ampere, 49, 54 

Amplification, 182; one stage of, 
185 

Amplitude of vibration, 155 

Antenna current variation, 141 

Arlington tests, 233 

Artificial telephone line, 252 

Atom, conception of, 6; nucleus 
of, 10; neutral, 34 

Atomic number, 13 

Atoms, difference between, 12; 
kinds of, 6, 10; motion of, 35 

Attenuation of current in wires^ 
259 

Audibility meter, 218 

Audio-frequency amplifier, 185; 
limitations of, 185 

Audion, 35, 40, 42 

Audion, amplifier, 182; detector, 
theory of, 126; modulator, 232; 
oscillator, theory of, 89; fre- 
quency control of, 99 

B-battery for tubes, 43; effect 
upon characteristic, 128 

Banked wound coils, 228 

Battery, construction of gravity, 
16; dry, 27; reversible or stor- 
age, 29 

Band of frequencies, 249 

Beat note, detection of, 221, 245 

Bell system, Arlington trans- 
mitter, 249 

Blocking of tube, reason for, 17 i 

Blue vitriol, 16 

Bridge circuit, 255 

Bureau of Standards, 50 



Capacity, effect upon frequency, 
100; measurement of, 104; 
unit of, 104; variable, 107 

Capacity effects, 243 ; elimination 
of, 228 

Carrier current, modulation of, 
146; telephony, 255 

Characteristic, of vacuum tube, 
68, 74; effect of B-battery 
upon, 128; how to plot a, 70 

Characteristic curve of trans- 
former, 64 

Chemistry, 8 

Choke coils, 210, 221 

Circuit, A, B, C, 187; coupled, 
115; defined, 43; oscillating, 
113; plate, 45; short, 30; tune 
of a, 117 

Condenser, defined, 77; charging 
current of, 78; discharge cur- 
rent of, 80; impedance of, 135; 
theory of, 78; tuning, 224 

Common battery system, 254 

Connection for wire to radio, 254 

Continuous waves, 86 

Copper, atomic number of, 13 

Copper sulphate, in solution, 21 

Crystals, atomic structure, 147 

Crystal detectors, 146; character- 
istic of, 148; circuit of, 150; 
theory of, 147 

Current, transient, 114; radio, 
144 

Cycle, 94, 97 

Damped oscillations, 114 
Demodulation, 231 
Detection, explained, 146 
Detectors, audion, 126; crystal, 

146 
Direct currents, 205 
Dissociation, 22 
Distortion, of wave form, 163 
Dry battery, 27 



C-battery for tubes, 46, 166; vari- Earth, atomic constitution, 11 

ation of, 75; for detection, 66 Effective value, of ampere, 207; 
Calibration of a receiver, 214 of volt, 207 

263 



264 



INDEX 



Efficiency, of regenerative circuit, 
182 

Electrical charge, 22 

Electricity, current of, 15, 16 

Electrodes, of vacuum tube, 41 ; 
definition of, 41 

Electrolyte, definition of, 34 

Electrons, properties of, 4; plan- 
etary, 10, 12; rate of flow, 48; 
vapor of, 39; wandering of, 14 

Electron streams, laws of attrac- 
tion, 200 

E. M. F., 59; alternating, 76; of 
self-induction, 238 

Energy, expended in tube, 235; 
of electrons, 113; radiation of, 
125 

Ether, 88 

Feed-back circuit, 182 

Frequency, 98, 158; effect upon 
pitch, 133; interval, 247; nat- 
ural, 117; of voice, 163 

Fundamental note, of string, 157 

Gravity battery, theory of, 23 
Grid, action of, 47; condenser, 

169; current, 173; leak, 171; 

leak, construction, 172, 216; 

of audion, 41 

Harmonics, 160 
Helium, properties of, 9 
Henry, 83 
Heterodyne, 181 
Hot-wire ammeter, 51 
Human voice, mechanism of, 152 
Hydrogen, action of in acid, 7; 
atom of, 7 

Impedance, of coil, 136; of con- 
denser, 136; of transformer, 
195; effect of iron core upon, 
207; matching of, 196 

Intermediate-frequency amplifica- 
tion, 242 

Inductance, defined, 83; effect 
upon frequency, 100; imped- 
ance of, 135; mutual, 109; of 
coils, 101; self, 83; table of 
values, 227; unit of, 83; vari- 
able, 108 

Induction, principle of, 208 

Inductometer, 109 

Input circuit, 187 



Interference, 249 

Internal resistance, 191 

Ion, definition of, 19; positive 

and negative, 20, 21 
Ionization, 20 

Larynx, 153 
Laws of attraction, 204 
Loading coil, 224 
Loop antenna, 198 

Magnet, pole of, 203; of soft 
iron, 205; of steel, 205 

Magnetism, 202 

Matter, constitution of, 5 

Megohm, 172 

Microfarad, 104 

Mil-ampere, 71 

Mil-henry, 83 

Modulation, 145, 230, 237, 239 

Molecule, kinds of, 6; motion of, 
35 

fiv, 190 

Multiplex telegraphy, 258; tele- 
phony, 258 

Mutual inductance, 109; varia- 
tion of, 110 

Natural frequency, 161 

Nitrogen, 10 

Nucleus of atom, 10, 12 

Ohm, defined, 64 
Organ pipe, 160 
Oscillations, 87; damped, 114; to 

start, 114; intensity of, 236; 

natural frequency of, 117 
Output circuit, 187 
Overtones, 159 
Oxygen, percentage in air, 10 

Phase, 180 

Plate, of an audion, 41 

Plunger type of instrument, 205 

Polarity of a coil, 204 

Power, defined, 234; electrical 

unit of, 235 
Proton, properties of, 4 

Radio current, modulation of, 

145 
Radio-frequency amplification, 

243; limitations, 243 
Radio-frequency amplifier, 186, 

198 



INDEX 



265 



Radio station connected to land 
line, 254 

Rating ot tubes, 235 

Reception, essential operations 
in, 235 

Regenerative circuit, 176; fre- 
quency of, 179 

Repeater stations, 261 

Resistance, measurement of, 64; 
non-inductive, 103; square, 251 

Resonance, 161 

Resonance curve, 117 

Retard coils, 210 

Salt, atomic construction of, 17; 
crystal structure, 147; mole- 
cule in solution, 19; percent- 
age in sea water, 11 

Saturation, 38 

Sea water, atomic constitution 
of, 11 

Self-inductance, 83; unit of, 83 

Side bands, 248 ; relation to wave 
lengths, 249 

Silicon, percentage in earth, 11 

Sodium chloride, in solution, 19 

Sound, production of, 152 

Speech, to transmit by radio, 230 

Speed of light, 122 

Standard cell, 58 

Storage battery, 28, 30 

Sulphuric acid, 22 

Super-heterodyne, 242 ; advan- 
tages of, 242 

Telephone receiver, 130; theory 

of, 131 
Telephone transmitter, 142 
Telephony, by wire, 253 
Tickler coil, 182 
Transcontinental telephone line, 

261 



Transmission, essential operations 

in, 230 
Transmitter, Arlington, 233; con- 
tinuous wave, 94, 119; for high 
power, 233 
Transformer, 185; step-up, 193 
Tubes, connected in parallel, 234 
Tuning, curve, 117; sharp, 214; 
with series condenser, 224 

Undamped waves (see continuous 
waves), 86 

Vacuum tube, 35, 40; character- 
istics of, 67 ; construction of, 
205; modulator, 239; three- 
electrode, 41 ; two-electrode, 42 

Variometer, 108 

Vibrating string, study of, 154 

Vocal cords, 153 

Voice frequencies, 163 

Volt, definition of, 57; measure- 
ment of, 61 

Voltmeter, calibration of, 62; 
construction of, 205 

Watt, 235 

Wave form, 182 

Wave length, relation to fre- 
quency, 98, 122; defined, 122 

Wire, inductance of, 104 

Wire, movement of electrons in, 
14; emission of electrons from, 
37 

Wire telephony, 253 

Wired wireless, 255; advantages 
of, 257 

X-rays, 147 

Zero coupling, 177 

Zinc, electrode for battery, 23 



JLiN -0 i942 



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